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Direct synthesis of carbon nanotube interpenetrated doped porous carbon alloy as a durable Pt-free electrocatalyst for oxygen reduction reaction in alkaline medium† Sreekuttan M. Unni a, Gopinathan M. Anilkumar b, d, Masashi Matsumoto c, d, Takanori Tamaki a, d, Hideto Imai c, d and Takeo Yamaguchi a, d* Direct synthesis of highly durable carbon nanotube interpenetrated porous carbon alloy electrocatalyst for oxygen reduction reaction (ORR) from a single precursor, trimetallic zeolitic imidazole framework (t-ZIF) is reported. The use of single precursor improves the uniform distribution of active reaction centres which is crucial for ORR catalysts. The t-ZIF has Fe, Co and Zn metal centres and 2-methylimidazole as a ligand. Carbonisation of t-ZIF under inert atmosphere produce nitrogen and Fe/Co-Nx doped carbon/carbon nanotube alloyed with metal/metal oxide particles encased inside the carbon structures (FeCo-NCZ). The presence of Zn in the t-ZIF induces porosity in carbon during the carbonisation process. The peculiar morphology with reasonably high surface area provides efficient mass transport and interpenetrated carbon nanotube assist fast electron transport in the catalyst. X-ray photoelectron spectroscopy reveals that FeCoNCZ is enriched with different possible active reaction centres such as pyridinic, graphitic and Fe/Co-Nx type nitrogen coordination on the catalyst surfaces. The ORR activity of FeCo-NCZ in oxygen saturated 0.1 M KOH comparable/higher than reference Pt/C catalyst. The displayed onset potential (1.04 V vs. RHE) and halfwave potential (0.91 V vs. RHE) of FeCo-NCZ is more positive compared to Pt/C and other control-samples. It is noteworthy that the dioxygen reduction kinetics of FeCo-NCZ is comparable to Pt/C as evident from Tafel slope and oxygen reduction follows four electron pathways. More interestingly, FeCo-NCZ shows better fuel tolerance and electrochemical stability even at 60 oC compared to that of Pt/C under alkaline condition.

Introduction Development of Pt-free electrocatalyst for oxygen reduction reaction (ORR) is an emerging research area for the cost-effective 1, 2 low-temperature fuel cell and metal-air batteries. Among the various electrocatalysts being developed, low-cost metal (Fe, Co, etc.) and non-metal (N, S, B, P, etc.) doped carbon morphologies gained more attention as an electrocatalyst for the oxygen 3-5 reduction reaction (ORR). Since the active reaction site density on the doped carbon plays a major role for the efficient reduction of oxygen, many synthetic strategies are being adopted to improve its 4, 6-8 active site densities. The general method for the catalyst

preparation comprises the high-temperature treatment of the 9 mixture of carbon, metal precursor and heteroatom sources. However, such method of preparation leads to electrocatalysts with unequal distribution of active centres on it.10 It may ultimately reflect in the electrocatalytic activity towards ORR and electrochemical stability of electrocatalysts. Carbonisation of a single precursor which contain all the prerequisite in a preferred chemically linked ordered structure will be an efficient strategy for the development of electrocatalysts with uniform active site density 11 on it. Metal organic framework (MOF) materials recently gained a great interest, as a precursor to the development of ORR electrocatalyst due to the peculiar morphology, high surface area, well-defined 12-14 pore structure, and preferred metal-ligand interaction. Among different MOFs, zeolitic imidazole framework (ZIF) is the most 14-20 studied candidate for the development of Pt-free ORR catalysts. Dodelet et. al., used ZIF-8 (zinc based imidazole framework) as carbon and nitrogen source for the electrocatalyst preparation.21 There are reports on the synthesis of electrocatalyst from ZIF-67 22 (cobalt based imidazole framework). However, additional metal or heteroatom sources is necessary to improve the ORR activity of 19 derived carbon. Further, David Lou et al. reported a variety of modification on ZIF-8 and ZIF-67 for different applications including 23-27 ORR. Later, bimetallic ZIF (both Zn and Co as metal centres)

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were introduced to improve the ORR activity. High-temperature annealing of such bimetallic ZIF results improved activity in alkaline

efficient active reaction centre without any further addition of iron coordinated molecules. Moreover, the presence of zinc will assist in-situ formation pores during high-temperature annealing through its evaporation at high temperature. The t-ZIF derived carbon catalyst (FeCo-NCZ) has a morphology of carbon nanotube interpenetrated porous carbon. It shows improved ORR activity in an alkaline medium which is comparable to commercial Pt/C.

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Results and discussion

Scheme 1. Schematic representation of the synthesis of carbon nanotube interpenetrated porous carbon alloy (FeCo-NCZ) electrocatalysts. 34

or acidic conditions towards ORR. During high-temperature annealing, Zn can evaporate and generate porosity in the final 28 carbon. It will lead to the improvement in the active reaction site density. Synthesis of such mono metallic and bimetallic ZIF usually follows a traditional time consuming, low yield synthesis process and impede bulk synthesis of the electrocatalysts. Recently Gross et.al. synthesised ZIF-8 and ZIF-67 through a low-temperature 35 aqueous phase approach and produced ZIF with high yield. However, the carbonised product from such ZIF showed poor ORR 36 activity in alkaline medium. Therefore, a proper synthetic strategy is required to achieve tailored carbon alloy catalyst with higher/better electrochemical properties. The fast synthesis of ZIF-8/ZIF-67 through aqueous room temperature process produces nanometre-sized ZIF and can accommodate the foreign metal ion by which the formation of ZIF is practically difficult. Thus, by adopting a fast room temperature aqueous synthesis procedure of ZIF can deliver the following benefits 1) combination of different metal centres, 2) high yield and 3) nanometre size particles. Shui et al. reported that the preferred iron-based coordination compound is necessary to tune ZIF-8 37, 38 derived catalyst to perform better ORR activity. It can be easily achieved by adopting fast aqueous synthesis approach where ZIF 39 can efficiently host the in-situ formed metal-ligand complex. Here, we report the synthesis of carbon electrocatalysts from Zn, Fe and Co based trimetallic ZIF (t-ZIF) using the fast synthesis procedure. According to Zelenay et.al., the combination of Co and Fe in the non-Pt catalysts improves the electrochemical reduction of 40 dioxygen. So the carbonisation of t-ZIF can leads such preferred coordination of both iron and cobalt. By adopting the fast synthesis procedure, iron metal ions coordinated with ligand molecule during the synthesis of ZIF can occupy in the pores of ZIF and deliver

Trimetallic (Zn, Co and Fe) ZIF was prepared using the procedure 35 reported by Gross et al. Aqueous solutions of the corresponding metal salts and imidazole/triethylamine solution were mixed and stirred for 30 min to prepare t-ZIF. Carbonisation of the as made tZIF was carried out at high temperature in the N2 atmosphere. After the carbonisation process, to remove surface metal oxide impurities, the carbon was further subjected to acid wash. A second o annealing at 910 C was performed on the acid washed carbon to get the final catalyst (Scheme 1) (detailed experimental procedure is given in the experimental section). The as made t-ZIF (from precursors of Co, Fe and Zn) was named as FeCoZn-ZIF, and the carbon alloy catalyst derived from the above ZIF denoted as FeCoNCZ. Control-samples were also prepared by changing the metal content during ZIF synthesis. The carbon alloy derived from Co-ZIF (ZIF-containing only Co), CoZn-ZIF (ZIF-containing Co and Zn) and FeZn-ZIF (ZIF-containing Fe and Zn) were named as Co-NC, Co-NCZ and Fe-NCZ, respectively. X-ray diffraction (XRD) patterns (Figure 1(a)) of Co-ZIF and CoZn-ZIF resembles similar peaks, indicating the structural feature in the ZIF is same. The XRD peaks of FeCoZn-ZIF and FeZn-ZIF show similar feature to that of Co-ZIF and CoZn-ZIF. It is a clear indication that the addition of iron is not affecting the crystalline features of Co-ZIF and CoZn-ZIF. During the ZIF synthesis, the imidazole coordinated iron is well packed in the available pores of the ZIF without altering its crystalline nature. The XRD patterns reveal a sodalite type crystal 35 structure for all prepared ZIF. After high-temperature annealing, Co-NC, and Co-NCZ shows diffraction peaks corresponding to that of metallic cobalt, carbide and oxide phases of cobalt (Figure 1 (b)). The peak at 2θ ~26 o corresponds to the (002) plane of graphitic o carbon. The peak at 2θ ~31.2, 36.9, 59.5 and 65.4 indicates the 41, 42 presence of crystalline Co3O4 phase of cobalt. The peak at o o 2θ~43.46 and 44.3 corresponds to the carbide and metallic form of cobalt respectively. XRD pattern of Fe-NCZ also shows the o presence of oxide (2θ~30.1, 35.4, 57.3, and 62.9 for Fe3O4), o o carbide (2θ~43.3 ) and metallic iron (2θ~44.8 ) phases, along with 43 (002) plane of graphitic carbon. More interestingly, XRD pattern of FeCo-NCZ displays more intense peak of metallic phases of Fe/Co o o (2θ~ 44.8 ). But the peak intensity of oxide phases of Co (2θ~65.4 ) and Fe (2θ~ 35.4, 57.3, and 62.9o) is less compared to metallic phases. It indicates the presence of more crystalline metallic forms of Fe/Co in the FeCo-NCZ. Raman spectra of the annealed ZIF show (Figure 1(c)) two dominant peaks at 1339.54 and 1585.30 cm-1 corresponds to D and G bands of the carbon. G band represent the graphitic (graphitic lattice vibrational mode with E2g symmetry), and D band represent the disordered graphite (graphitic lattice vibration mode with A1g symmetry) form of carbon.44 In the annealed catalyst, the D band intensity is higher compared to G band indicates the presence of more disordered carbon in the annealed catalyst. The graphitization degree was calculated using the intensity ratio of D and G bands. The ID/IG ratio of catalyst is in the order of Fe-NCZ (1.55) > Co-NC

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Figure 1. XRD pattern of the prepared ZIFs (a) and ZIF derived carbon (b), (c) Raman spectra of ZIF derived catalysts and (d) Nitrogen adsorption-desorption isotherm of different carbon catalysts (Inset: the pore size distribution of different carbon catalysts)

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(calculated using BJH method) is in the order of Fe-NCZ (1.24 cm /g) 3 3 > Co-NC (1.15 cm /g) > Co-NCZ (1.13 cm /g) >FeCo-NCZ (1.05 3 cm /g). Since micropores contribute major role towards the improvement in the active reaction site density, the micropores 2 surface area was calculated. FeCo-NCZ (262 m /g) and Co-NCZ (280 2 m /g) display high micropore surface area compared to Fe-NCZ and Co-NCZ. Scanning electron microscopic (SEM) images of FeCoZn-ZIF and CoZIF show similar morphological features with an average particles size of 60 nm (Figure S1†). However, aMer carbonisaNon morphological features of ZIF were collapsed. More evidence about the morphology of annealed carbon from different ZIFs was obtained from transmission electron microscopic images (TEM). As seen in figure 2, the FeCo-NCZ has a morphology of carbon nanotube interpenetrated porous carbon with metal/metal oxide particles embedded in it. The porosity can be created during the acid leaching of metal/metal oxide nanoparticles in addition to the evaporation of Zn from the ZIF. A disordered carbon fringes on the carbon mass are clearly indicating the effective doping of 50 heteroatoms. The metal/metal oxide nanoparticles are well encased in the carbon shells. The nanometer thick graphene type carbon shell around the metal/metal oxide nanoparticles prevent it from leaching out during the acid washing process. As reported by 51 Gewirth et al., these metal nanoparticles also capable of activating the doped carbon which covered the nanoparticles to act as an active centre for oxygen reduction. This possibility is theoretically 52 explained by Deng et al.

catalyst systems. The ID/IG ratio assist to calculate the in-plane crystallite size (La) of the annealed carbon.45 The La of the carbon catalyst is in the order of FeCo-NCZ (17.3 nm) > Co-NCZ (16.7 nm) > Co-NC (15.9 nm) > Fe-NCZ (12.4 nm). The improved crystallite size of FeCo-NCZ represents the reduced resistivity, which is one of the prerequisites for an efficient electrocatalysts. The in situ formed carbon nanotubes in FeCo-NCZ also contribute to enhance the graphitization and leads to better electrical conductivity of the 46, 47 Apart from G and D bands, the peaks correspond to material. the transverse and longitudinal optical phonon mode vibration of the oxide of Co (for Co-NCZ and Co-NC) and Fe (for Fe-NCZ) observed in between 150-750 cm-1. This indicates the presence of 48 the oxide of cobalt and iron on the surface of annealed catalysts. The less intense peaks of the oxides of cobalt and iron in cases of FeCo-NCZ suggest that those nanoparticles are well encased in the carbon layers. The surface area of the prepared electrocatalyst was analysed by nitrogen adsorption-desorption isotherms using BrunauerEmmet-Teller (BET) method at 77 K. Nitrogen adsorptiondesorption isotherm of electrocatalysts resemble a type I isotherm with more microporous features (Figure 1 (d)). The surface area of the electrocatalysts calculated from BET is in the order of Co-NCZ 2 2 2 (470 m /g)>FeCo-NCZ (463 m /g) >Fe-NCZ (393 m /g) >Co-NC (340 2 m /g). Co-NCZ and FeCo-NCZ show almost similar surface area. As expected, the Co-NC has a lower surface area as compared to that of the other ZIF derived carbon. A slight decrease in the surface area of Fe-NCZ in comparison to FeCo-NCZ and Co-NCZ is mainly due to the difference in the nature of carbon derived from the 49 FeZn-ZIF compared to the carbon from FeCoZn-ZIF and CoZn-ZIF. The pore-size distribution measured using Horvath-Kawazoe (HK) method shows the pore dimeter of all catalyst is in between 0.5 - 1 nm (inset of Figure 1 (d)). Pore volume of the electrocatalyst

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(1.21) > Co-NCZ (1.15) > FeCo-NCZ (1.11). This clearly indicates that the FeCo-NCZ has better graphitic phase compared to the other prepared

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Along with porous carbon mass, TEM images also reveal several micrometre length bamboo shaped carbon nanotubes, which are uniformly distributed throughout the carbon. Since Fe catalyse the nanotube formation from small organic molecules, the nanotube in the present case formed from the small organic molecules derived 43 from the ZIF at high temperature. The uniform distribution of nanotubes in the carbon matrix further improves the effective electron transport through it. The elemental mapping (Figure 2e) clearly shows that all elements are well dispersed in the carbon matrix of FeCo-NCZ. The mapping of Fe and Co clearly depict the uniform distribution over the surface, and the position of these metal centres are exactly matching on the carbon matrix resembling an alloy type feature. The uniform distribution of these metal centres on carbon surface assist the formation of improved active reaction centres and make FeCo-NCZ as better electrocatalysts for ORR. Similar morphological features were also observed for the Fe-NCZ (Figure S2†). However, the morphology of Fe-NCZ resembles a densely packed carbon mass and carbon nanotube. It may be the reason for the reduced surface area of FeNCZ compared to FeCo-NCZ and Co-NCZ. The HRTEM images of CoNC (Figure S3†) and Co-NCZ (Figure S4†) shows porous nature of carbon morphology with nanoparticles dispersed in it. More interestingly, the bamboo shaped nanotubes were absent in these carbon structures. This indicates that the Co particles in the present case are not producing carbon nanotubes during carbonisation process. Interestingly, most of the nanoparticles are well encased in the carbon shells.

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(1020.50 eV). Atom percentage (at.%) of all elements present in the prepared electrocatalysts are given in Table S1. The deconvoluted N 1s spectrum (Figure 3) of all catalyst display six different type of nitrogen coordination such as pyridinic (~398.22 eV), M-Nx (~399.30 eV), pyrrolic (400.30 eV), graphitic (401.10 eV), pyridine N-oxide (~402.61 eV) and π-π* shake-up/N2 (~403.90 54 eV). The pyridinic and graphitic nitrogen coordination in doped carbon materials improve the reduction of oxygen by improving the 55, 56 onset potential and reduction current. The at.% of pyridinic nitrogen in the catalyst is in the order Co-NCZ (41.4%) > FeCo-NCZ (41.0 %) > Fe-NCZ (34.4 %)> Co-NC (29.3 %). FeCo-NCZ and Co-NCZ have comparable amount of pyridinic nitrogen. Similarly, at.% of graphitic nitrogen content is in the order of Co-NC (25.0 %) >Fe-NCZ (23.6 %)> FeCo-NCZ (20.9 %)> Co-NCZ (14.3 %). However, the sum of both pyridinic and graphitic nitrogen content is higher for FeCoNCZ and is expect to improve the reaction site density. The companied effect of both coordination reflects on the enhancement of ORR of FeCo-NCZ in comparison with other catalysts. The M-Nx interaction in Co-NCZ and Co-NC found to be 15.7 and 17.8 at.% respectively. Since trace amount of Zn is also present in the Co-NCZ, Zn-Nx can also contribute to the total M-Nx of Co-NCZ. The Fe-Nx interaction in Fe-CNZ is calculated to be 16.6 at.%. The at.% of nitrogen in M-Nx type interaction in FeCo-NCZ is 11.62. The ratio of surface nitrogen to carbon (N/C) is higher for the FeCo-NCZ. The N/C ratio of the electrocatalyst is in the order of FeCo-NCZ (0.065) >Co-NCZ (0.061) > Fe-NCZ (0.057) >Co-NC (0.023). The higher N/C ratio of FeCo-NCZ indicates that the surface of FeCoNCZ is enriched with preferred nitrogen coordination to improve the active reaction site density. The at.% of different nitrogen coordination is given in Table 1.

Table 1: Atom percentage of different nitrogen coordination in the electrocatalysts, calculated from XPS data. Pyridinic M-Nx Pyrrolic Graphitic Pyridine Noxide π-π* shakeup/N2 Total nitrogen content Figure 3. Deconvoluted XPS spectrum N1s of (a) FeCo-NCZ, (b) FeNCZ, (c) Co-NCZ and (d) Co-NC Different elemental composition and its chemical environment were analysed using X-ray photoelectron spectroscopy (XPS). All annealed ZIF samples show the presence of carbon, nitrogen, and oxygen. Cobalt is present in FeCo-NCZ, Co-NCZ and Co-NC. XPS demonstrates the presence of iron in FeCo-NCZ and Fe-NCZ. More interestingly, FeCo-NCZ and Co-NCZ shows trace amount of zinc in it. The existence of residual zinc indicates incomplete evaporation. Deconvoluted XPS spectrum of Zn 2p (Figure S5†) shows the presence of three different types of zinc in FeCo-NCZ and Co-NCZ. They are Zn-O (1021.73 eV), Zn-N (1022.97 eV), and Metallic Zn

FeCo-NCZ 41.0 11.7 20.3 20.9

Fe-NCZ 34.4 16.6 17.2 23.6

Co-NCZ 41.4 15.7 21.3 14.4

Co-NC 29.3 17.8 22.7 25.0

3.8

5.5

4.9

5.2

2.3

2.7

2.3

0.0

5.65

3.32

5.22

2.10

Deconvoluted XPS spectrum of cobalt (Figure S6†) shows the presence of metallic Co (778.39 eV), Co-Nx (783.93 eV) and oxide of 57 cobalt (779.83 eV) in Co-NC, Co-NCZ and FeCo-NCZ. Among these three different coordinations, Co-Nx plays a crucial role to act as 58 ORR active centre. The at.% of cobalt in Co-Nx type coordination is almost similar (25.6 %) in FeCo-NCZ, Co-NCZ and Co-NC. The uniform distribution of these Co metal centres may alter the physical properties associates with the ORR of the electrocatalysts. Similarly, the deconvoluted Fe 2p spectrum also indicates the presence of Feo, FeII, FeIII forms of iron. In Figure S7†, the peaks at 710.10 and 711.92 eV correspond to the 2p3/2 orbital of Fe(II) and Fe(III), respectively. The peaks at 723.57 and 725.81 eV correspond 59 to 2p1/2 orbitals of Fe(II) and Fe(III), respectively. Fitted Fe 2p spectra of Fe-CNZ also show similar kinds of peaks. Two additional

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Figure 2. TEM images of FeCo-NCZ at different magnifications((a) 100 nm, (b) 20 nm, and (c) 5 nm). (d) STEM image of FeCo-NCZ and (e) represent the elemental mapping of FeCo-NCZ.

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The electrocatalytic activity of the carbon alloy catalysts towards the reduction of dioxygen molecules was analysed using linear sweep voltammetry (LSV) technique in oxygen saturated 0.1M KOH -1 at a scan rate of 10 mVs . Hg/HgO and Pt were used as a reference electrode and counter electrode respectively. Measured potential is

Figure 4. (a) Linear sweep voltammogram (LSV) of all electrocatalyst in oxygen saturated 0.1 M KOH at an electrode rotation of 1600 -1 rpm with a scan rate of 10 mV s . The anodic scan is used for the measurement of voltammogram. The disc current is normalised -2 with active electrode area (0.196 cm ) of the glassy carbon electrode. (b) The Tafel plot of FeCo-NCZ and Pt/C at low overpotential region, (c) peroxide yield and (d) electron transfer number of the electrocatalysts at different electrode potentials. represented in reversible hydrogen electrode (RHE) by calibrating Hg/HgO in hydrogen saturated 0.1 M KOH (figure S9†). Figure 4a shows the LSV of the electrocatalyst. From the figure, it is clear that FeCo-NCZ display improved ORR activity compared to Pt/C and other control-samples. FeCo-NCZ shows an onset potential of 1.04 V and a half-wave potential at 0.91 V. More interestingly, Pt/C experiences 10 mV higher overpotential compared to FeCo-NCZ in onset potential and half-wave potential. Among different electrocatalysts synthesised, Co-NC displays poor ORR activity in term of onset (0.92 V) and half-wave potential (0.82 V). Fe-NCZ and Co-NCZ display almost similar performance and Co-NCZ display an overpotential of 10 mV in half-wave potential compared to FeNCZ. Fe-NCZ and Co-NCZ experience 50 mV and 60 mV respectively in half-wave potential compared to FeCo-NCZ. The Tafel plot (The plot of E vs. log|-jk|) of the FeCo-NCZ and Pt/C shows a Tafel slope of 87.4 mV decade-1 and 84.9 mV decades-1 respectively (Figure 4b). It indicates that the dioxygen reduction mechanism of both Pt/C and FeCo-NCZ is almost similar in 0.1 M KOH solution. Further, the mass activity of the FeCo-NCZ and Pt/C (mass of both carbon and Pt) were measured at the potential of 0.95 V, and it found that the

mass activity of FeCo-NCZ is 1.6 times less compared to Pt/C. The further modifications in the catalyst to improve its mass activity is one of the ongoing researches in our group. The hydrogen peroxide yield (Mol %) and the electron transfer number during ORR was further measured using RRDE. 

   %   

 

  





      Where ‘ir’ is ring current, ‘id’ is disc current, ‘N’ is collection efficiency and ‘n’ is the number of electron transfer during ORR. Figure 4c shows the peroxide yield of all electrocatalysts. The calculated hydrogen peroxide yield of all electrocatalyst is below 10 Mol %. At a potential of 0.4 V, FeCo-NCZ display peroxide yield of 2.25 Mol %. The peroxide yield of the non-Pt catalyst at 0.4 V is in the order of FeCo-NCZ< Co-NC< Co-NCZ< Fe-NCZ. The number of electron transfer (Figure 4d) during ORR is around to be ~ 3.9 in all four non-Pt catalysts, indicate that ORR follows a direct four electron transfer pathways. i.e., the reduction of oxygen molecule produces OH ions instead of HO2 . Excellent stability of electrocatalyst at fuel cell working temperature is one of the significant concern in the catalyst development. Electrochemical stability of FeCo-NCZ was measured by potential cycling (load cycle) between 0.6 V and 1 V for 10k cycles at a o temperature of 60 C in nitrogen saturated 0.1 M KOH. LSV was taken before and after potential cycling in oxygen saturated 0.1 M KOH at room temperature. This cycling process assists to accelerate the degradation of the carbon catalyst. The cycling stability of Pt/C was also carried in the similar experimental conditions. FeCo-NCZ showed 37 mV negative shift in half-wave potential after 10k potential cycles, however, in the case of commercial Pt/C, the negative shift was 67 mV after 10k potential cycles. It clearly demonstrates that FeCo-NCZ is highly durable compared to Pt/C o even at a temperature of 60 C. Analysis of the TEM images (Figure S10†) performed after the durability analysis of FeCo-NCZ shows that FeCo-NCZ is free from severe morphological changes even after o 10k potential cycles. The excellent durability at 60 C may be attributed to the presence of high density of active reaction centre on carbon surfaces. But in the case of Pt/C, sintering or dissolution of Pt nanoparticles may occurs during the potential cycling which 62 causes the reduction in the catalyst performance. The fuel tolerance of the non-Pt electrocatalyst also studied in oxygen saturated 0.1 M KOH with and without methanol. Figure S11a† shows the LSV of FeCo-NCZ is unaltered with the addition of methanol. It indicates that the carbon alloy catalyst (non-Pt) is high tolerance towards methanol. It shows that the fuel like methanol cannot affect the ORR activity of FeCo-NCZ catalyst. But the ORR activity is drastically changed with the addition of methanol in the case of Pt/C. This further validates the applicability of FeCo-NCZ in direct methanol alkaline fuel cells.

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peaks at 707.72 and 714.46 eV corresponds to FeO and Fe-Nx 43 respectively were also observed in FeCo-NCZ and Fe-NCZ. Among different iron coordination, Fe-Nx contributes as active centre to 60 improves the ORR activity of the electrocatalysts. XPS analysis provides clear evidence that all the preferred coordination which can enhance the electrocatalytic reduction of oxygen is available for FeCo-NCZ. Deconvoluted C1s spectrum (Figure S8†) of all synthesised electrocatalysts shows four distinct peaks corresponding to binding energies 284.3 eV (C=C), 285.4 eV (C-N), 61 286.7 eV (C-O), and 288.5 (C=O).

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More positive onset potential, as well as the half-wave potential of FeCo-NCZ, compared to Pt/C and other control-carbon catalysts is mainly due to the enriched possible active reaction centres such as graphitic, pyridinic and Fe/Co-Nx type coordination on the surface of FeCo-NCZ. The uniform distribution of these active reaction centres is achieved only through adopting a single precursor for the preparation of electrocatalyst using a high-temperature annealing process. The reasonably high microporous surface area of FeCoNCZ improves the distribution of active reaction centre on the carbon surfaces and assist the reduction of dioxygen more efficiently. This high density of active reaction centres on the surface of FeCo-NCZ together with the reduced defective sites (< ID/IG) further helps to improve the electrochemical stability of FeCoNCZ. It is clearly evidenced by the 37 mV negative shift in half-wave potential after 10 k potential cycles in comparison with Pt/C. The peculiar morphology of the FeCo-NCZ plays a significant role in improving the catalytic activity. The carbon nanotube interpenetrated porous carbon assist the efficient mass transport as well as the electron transport throughout the catalyst. The high crystallite size of the FeCo-NCZ indicates low resistivity in FeCo-NCZ. These results emphasise that the FeCo-NCZ is a better electrocatalyst for oxygen reduction in solid alkaline fuel cells.

Experimental Materials 2-Methylimidazole were purchased from Sigma Aldrich. Iron Sulphate heptahydrate (99%), Cobalt Nitrate Hexahydrate (99.5%), Zinc Nitrate Pentahydrate (99%), Triethylamine were purchased from Wako pure chemical Industries (Japan). All chemicals were used as received. Synthesis of carbon alloy electrocatalysts In a typical experiment, 3.242g of 2-methyl imidazole was dissolved in 50 ml DI water, and 4g triethyl ammine (TEA) was added into the imidazole solution. The resulting mixture kept for stirring for 5 m. 0.55g zinc nitrate, 0.089 g cobalt nitrate, and 0.085 g iron sulphate dissolved separately in 50 ml DI water and added to the imidazoleTEA mixture with constant stirring. The stirring was continued for 30 min at room temperature. The resulting precipitate was centrifuged and washed several times with water. The prepared ZIF kept for drying in an oven at a temperature of 90 oC for 24 h. The ZIF contain 75% Zn and 25 % Fe and Co in a ratio of 1:1. The electrocatalysts from the above-synthesised ZIF were prepared by o carbonising it at a high temperature (910 C) for one hour in an inert atmosphere (Nitrogen). After carbonising, the samples were o subjected to acid wash for 8 h at 80 C using 2 M H2SO4, followed by washing with DI water for several times. The resulting carbon dried o o overnight at 90 C and annealed again at a temperature of 910 C for 0.5 h in the nitrogen atmosphere to get the final catalysts. For comparison purpose, catalyst from ZIF with different metal ion also prepared using similar synthesis procedure as described above. The details of the samples are given in Table 2.

Table 2: Details of various catalyst samples prepared from different ZIFs Catalyst

ZIF

FeCo-NCZ

FeCoZn -ZIF

Fe-NCZ

FeZn-ZIF

Co-NCZ

CoZn-ZIF

Co-NC

Co-ZIF (ZIF-67)

Materials Characterization XRD data obtained using an Ultima IV (Rigaku) with a Cu Kα (λ = 1.5406 Å) X-ray source operating at 40 kV and 40 mA and a scanning rate of 3° min−1. Raman analysis performed using LabRAM HR Evolution Raman spectrometer using a visible laser beam of wavelength 532 nm. BELSORP-Max (Microtrac BEL Corp. Japan) was used for surface area analysis. The analysis was carried at 77 K using ultrapure nitrogen gas. The morphology of the catalysts analysed using high-resolution transmission electron microscope (HRTEM) (images was taken using TOPCON EM-002BF-J operated in an accelerating voltage of 200 kV with Twin EDS facility) and the scanning electron microscope (S-4800, Hitachi HighTechnologies Corporation). The surface elemental composition of the carbon catalyst was analysed using Quantum 2000 (ULVAC-PHI Inc., Japan) using an X-ray source of monochromatic Al Kα (hν = 1486.58 eV) with photoelectron takeoff angle of 45° and X-ray irradiation area of 100 μmΦ. Electrochemical Characterization The electrochemical analysis was carried in electrochemical measurement system HZ-7000 and Dynamic Electrode HR-301 (HD HOKUTO DENKO). Oxygen reduction reaction (ORR) activity of the electrocatalyst was evaluated using linear sweep voltammetry (LSV) with an electrode rotation of 1600 rpm at a scan rate of 10 mV s-1 in oxygen saturated 0.1 M KOH solution. The Hg/HgO (1M KOH) and Pt have used reference and counter electrode respectively. The 2 electrocatalyst coated glassy carbon electrode (0.196 cm ) was used as working electrode. The potential was converted to reference hydrogen electrode (RHE) and expressed in RHE. The calibration of Hg/HgO was performed using the method reported by Dai et al.63 For calibration, 0.1 M KOH is saturated with hydrogen by bubbling pure hydrogen for 30 minutes. Pt disc was used as working electrode. Hg/HgO and Pt were used as reference and counter electrode respectively. LSV was performed at a scan rate of 1 mV s1 . The potential was measured where the LSV curve crosses zero current. This potential was used for calibrating Hg/HgO to RHE. The catalyst slurry was prepared by sonicating the mixture of 20 mg active material, 80 µl 5 wt.% Nafion and 2 ml 3:1 water: IPA mixture for 3 h in a bath sonicator. 10 µl of the resulting slurry was drop coated on the glassy carbon electrode. The catalyst loading of non-2 Pt carbon catalysts is maintained as 0.5 mg cm on the electrode surface. LSV was performed in a anodic scan. To eliminate the capacitive current from the oxygen reduction current, the blank correction was carried to present the LSV data. For blank correction, LSV in the nitrogen saturated 0.1 M KOH was performed without electrode rotation. For comparison purpose, ORR activity of -2 Pt/C (46.1 wt.%, with Pt loading 30 µg cm ) also evaluated. The hydrogen peroxide yield was evaluated by performing rotating ring disc electrode (RRDE) in oxygen saturated 0.1 M KOH solution at an electrode rotation of 1600 rpm. The ring potential was kept at 0.5 V

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Figure 5. LSVs of (a) FeCo-NCZ and (b) Pt/C, before and after 10000 potential cycles in oxygen saturated 0.1 M KOH at an electrode rotation of 1600 rpm with the scan rate of 10 mV s-1. (For durability test, 10000 load cycles in between 1 V and 0.6 V was performed in nitrogen saturated 0.1 M KOH at 60 oC)

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Electrochemical stability of FeCo-NCZ was measured by potential cycling (load cycle) between 0.6 V and 1 V for 10000 cycles at a o temperature of 60 C in nitrogen saturated 0.1 M KOH. LSV was taken before and after potential cycling in oxygen saturated 0.1 M KOH at room temperature. In one cycle, electrode initially kept at 0.6 V for 3s and then changes to 1V for another 3 s. The electrolyte o solution temperature was kept as 60 C, and nitrogen gas was purged throughout the cycling process. For methanol tolerance study, LSV was taken before and after the addition of methanol (2mL) in 0.1 M oxygen saturated 0.1 M KOH (310 mL) at a scan rate -1 of 10 mV s with an electrode rotation of 1600 rpm.

Conclusions We presented a direct synthesis approach for the preparation of carbon nanotube interpenetrated porous carbon alloy from a single precursor, t-ZIF for ORR in alkaline medium. An aqueous rapid synthesis approach is used for the preparation of t-ZIF in high yield. t-ZIF consist of Fe, Co and Zn-based metal centres and 2-methyl imidazole as a ligand. On carbonisation, t-ZIF is transformed to carbon catalyst. HR-TEM reveals that the morphology of carbon nanotube interpenetrated porous carbon with metal/metal oxide nanoparticles are well encased in the carbon shells. Further, EDS analysis provides clear evidence for the uniform distribution of metal atoms and nitrogen moieties on the surfaces. Since the position of Co and Fe in the EDS images positioned in the same location and it indicates alloys type feature of the metal centres in the catalysts. XRD analysis shows FeCo-NCZ has metallic Fe/Co phases along with very less amount of oxide phases. BET 2 surface area of FeCo-NCZ is found to be 463 m /g with microporous features. XPS analysis reveals that the pyridinic, graphitic and Fe/Co-Nx type nitrogen coordination is dominated in the FeCo-NCZ compared to the other controlcatalyst prepared in the same process. This nitrogen coordination is important for efficient ORR activity, for Pt-free electrocatalysts. The ORR activity of FeCo-NCZ in oxygen saturated 0.1 M KOH is comparable to the commercial Pt/C. The displayed onset potential (1.04 V vs. RHE) and half-wave potential (0.91 V vs. RHE) of FeCo-NCZ is more positive compared to Pt/C and other control-samples. The dioxygen reduction kinetics of FeCo-NCZ is comparable to Pt/C as evident from Tafel slope, and oxygen reduction follows four electron pathways. More interestingly, FeCo-NCZ shows better o fuel tolerance and electrochemical stability even at 60 C compared to that of Pt/C. With improved ORR activity and better electrochemical stability, FeCo-NCZ can be a costeffective electrocatalyst for solid alkaline fuel cells.

Acknowledgements Authors acknowledge the financial assistance from Japan Society for the Promotion of Science (JSPS) and Core Research for Evolutionary Science and Technology, Japan Science and

Technology Agency (JST-CREST), Japan. SMU acknowledge JSPS for the research fellowship.

Notes and references 1. R. Cao, J.-S. Lee, M. Liu and J. Cho, Adv. Energy Mater., 2012, 2, 816-829. 2. D. C. Higgins and Z. Chen, Can. J. Chem. Eng., 2013, 91, 18811895. 3. Z. Yang, J. Ren, Z. Zhang, X. Chen, G. Guan, L. Qiu, Y. Zhang and H. Peng, Chem. Rev., 2015, 115, 5159-5223. 4. L. Dai, Y. Xue, L. Qu, H.-J. Choi and J.-B. Baek, Chem. Rev., 2015, 115, 4823-4892. 5. F. Jaouen, E. Proietti, M. Lefevre, R. Chenitz, J.-P. Dodelet, G. Wu, H. T. Chung, C. M. Johnston and P. Zelenay, Energy Environ. Sci., 2011, 4, 114-130. 6. X.-K. Kong, C.-L. Chen and Q.-W. Chen, Chem. Soc. Rev., 2014, 43, 2841-2857. 7. Y. Nie, L. Li and Z. Wei, Chem. Soc. Rev., 2015, 44, 2168-2201. 8. C. Zhu, H. Li, S. Fu, D. Du and Y. Lin, Chem. Soc. Rev., 2016, DOI: 10.1039/c5cs00670h. 9. H. Chung, G. Wu, D. Higgins, P. Zamani, Z. Chen and P. Zelenay, in Electrochemistry of N4 Macrocyclic Metal Complexes: Volume 1: Energy, eds. J. H. Zagal and F. Bedioui, Springer International Publishing, Cham, 2016, DOI: 10.1007/978-3-319-31172-2_2, pp. 41-68. 10. Z. Li, G. Li, L. Jiang, J. Li, G. Sun, C. Xia and F. Li, Angew. Chem. Int. Ed., 2015, 54, 1494-1498. 11. K. Chen, Y. Hao, M. Zhang, D. Zhou, Y. Cao, Y. Wang and L. Feng, RSC Adv., 2017, 7, 5782-5789. 12. W. Wang, X. Xu, W. Zhou and Z. Shao, Adv. Sci., 2017, 4, 1600371-n/a. 13. Q.-L. Zhu, W. Xia, T. Akita, R. Zou and Q. Xu, Adv. Mater., 2016, 28, 6391-6398. 14. W. Xia, A. Mahmood, R. Zou and Q. Xu, Energy Environ. Sci., 2015, 8, 1837-1866. 15. H. M. Barkholtz and D.-J. Liu, Mater. Horiz., 2017, 4, 20-37. 16. T. Palaniselvam, B. P. Biswal, R. Banerjee and S. Kurungot, Chem. Eur. J, 2013, 19, 9335-9342. 17. P. Zhang, F. Sun, Z. Xiang, Z. Shen, J. Yun and D. Cao, Energy Environ. Sci., 2014, 7, 442-450. 18. D. Kim, D. W. Kim, W. G. Hong and A. Coskun, J. Mater. Chem. A, 2016, 4, 7710-7717. 19. M. Thomas, R. Illathvalappil, S. Kurungot, B. N. Nair, A. A. P. Mohamed, G. M. Anilkumar, T. Yamaguchi and U. S. Hareesh, ACS Appl. Mater. Interfaces, 2016, 8, 29373-29382. 20. S. Pandiaraj, H. B. Aiyappa, R. Banerjee and S. Kurungot, Chem. Commun., 2014, 50, 3363-3366. 21. E. Proietti, F. Jaouen, M. Lefèvre, N. Larouche, J. Tian, J. Herranz and J.-P. Dodelet, Nat. Commun., 2011, 2, 416. 22. W. Xia, J. Zhu, W. Guo, L. An, D. Xia and R. Zou, J. Mater. Chem. A, 2014, 2, 11606-11613. 23. B. Y. Xia, Y. Yan, N. Li, H. B. Wu, X. W. Lou and X. Wang, Nat. Energy, 2016, 1, 15006. 24. H. Hu, L. Han, M. Yu, Z. Wang and X. W. Lou, Energy Environ. Sci., 2016, 9, 107-111. 25. B. Y. Guan, L. Yu and X. W. Lou, Energy Environ. Sci., 2016, 9, 3092-3096. 26. H. Hu, B. Guan, B. Xia and X. W. Lou, J. Am. chem. Soc., 2015, 137, 5590-5595.

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vs. Hg/HgO (1.39 V vs. RHE). The collection efficiency of the ring electrode was measured as 0.37 for the peroxide yield and electron transfer calculation.

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27. L. Zhang, H. B. Wu and X. W. Lou, J. Am. chem. Soc., 2013, 135, 10664-10672. 28. Y.-Z. Chen, C. Wang, Z.-Y. Wu, Y. Xiong, Q. Xu, S.-H. Yu and H.-L. Jiang, Adv. Mater., 2015, 27, 5010-5016. 29. M. Wang, T. Qian, J. Zhou and C. Yan, ACS Appl. Mater. Interfaces, 2017, 9, 5213-5221. 30. J. Tang, R. R. Salunkhe, H. Zhang, V. Malgras, T. Ahamad, S. M. Alshehri, N. Kobayashi, S. Tominaka, Y. Ide, J. H. Kim and Y. Yamauchi, Sci. Rep., 2016, 6, 30295. 31. S. Gadipelli, T. Zhao, S. A. Shevlin and Z. Guo, Energy Environ. Sci., 2016, 9, 1661-1667. 32. X. Wang, H. Zhang, H. Lin, S. Gupta, C. Wang, Z. Tao, H. Fu, T. Wang, J. Zheng, G. Wu and X. Li, Nano Energy, 2016, 25, 110119. 33. B. You, N. Jiang, M. Sheng, W. S. Drisdell, J. Yano and Y. Sun, ACS Catal., 2015, 5, 7068-7076. 34. L. Chong, G. A. Goenaga, K. Williams, H. M. Barkholtz, L. R. Grabstanowicz, J. A. Brooksbank, A. B. Papandrew, R. Elzein, R. Schlaf, T. A. Zawodzinski, J. Zou, S. Ma and D.-J. Liu, ChemElectroChem, 2016, 3, 1541-1545. 35. A. F. Gross, E. Sherman and J. J. Vajo, Dalton Trans., 2012, 41, 5458-5460. 36. M. Jiang, X. Cao, D. Zhu, Y. Duan and J. Zhang, Electrochim. Acta, 2016, 196, 699-707. 37. J. Shui, C. Chen, L. Grabstanowicz, D. Zhao and D.-J. Liu, Proc. Natl. Acad. Sci. U.S.A., 2015, 112, 10629-10634. 38. J. Tian, A. Morozan, M. T. Sougrati, M. Lefèvre, R. Chenitz, J.-P. Dodelet, D. Jones and F. Jaouen, Angew. Chem. Int. Ed., 2013, 52, 6867-6870. 39. Q. Lai, L. Zheng, Y. Liang, J. He, J. Zhao and J. Chen, ACS Catal., 2017, 7, 1655-1663. 40. G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 2011, 332, 443-447. 41. Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nat. Mater., 2011, 10, 780-786. 42. L. Tan, Y.-D. Yang, N. Li, S. Chen and Z.-Q. Liu, Catal. Sci. Tech., 2017, 7, 1315-1323. 43. S. M. Unni, R. Illathvalappil, S. N. Bhange, H. Puthenpediakkal and S. Kurungot, ACS Appl. Mater. Interfaces, 2015, 7, 2425624264. 44. S. K. Singh, V. M. Dhavale and S. Kurungot, ACS Appl. Mater. Interfaces, 2015, 7, 442-451. 45. S. M. Unni, S. Devulapally, N. Karjule and S. Kurungot, J. Mater. Chem., 2012, 22, 23506-23513. 46. W. Yang, X. Liu, X. Yue, J. Jia and S. Guo, J. Am. chem. Soc., 2015, 137, 1436-1439. 47. S. Dou, X. Li, L. Tao, J. Huo and S. Wang, Chem. Commun., 2016, 52, 9727-9730. 48. B. Li, X. Ge, F. W. T. Goh, T. S. A. Hor, D. Geng, G. Du, Z. Liu, J. Zhang, X. Liu and Y. Zong, Nanoscale, 2015, 7, 1830-1838. 49. H. Zhong, Y. Luo, S. He, P. Tang, D. Li, N. Alonso-Vante and Y. Feng, ACS Appl. Mater. Interfaces, 2017, 9, 2541-2549. 50. T. Palaniselvam, R. Kannan and S. Kurungot, Chem. Commun., 2011, 47, 2910-2912. 51. J. A. Varnell, E. C. M. Tse, C. E. Schulz, T. T. Fister, R. T. Haasch, J. Timoshenko, A. I. Frenkel and A. A. Gewirth, Nat. Commun., 2016, 7, 12582. 52. D. Deng, L. Yu, X. Chen, G. Wang, L. Jin, X. Pan, J. Deng, G. Sun and X. Bao, Angew. Chem. Int. Ed., 2013, 52, 371-375. 53. Y. Yuan, L. Yang, B. He, E. Pervaiz, Z.-G. Shao and M. Yang, Nanoscale, 2017, DOI: 10.1039/c7nr02264f.

54. M. E. M. Buan, N. Muthuswamy, J. C. Walmsley, D. Chen and M. Rønning, ChemCatChem, 2017, DOI: 10.1002/cctc.201601585, n/a-n/a. 55. L. Lai, J. R. Potts, D. Zhan, L. Wang, C. K. Poh, C. Tang, H. Gong, Z. Shen, J. Lin and R. S. Ruoff, Energy Environ. Sci., 2012, 5, 7936-7942. 56. X. Cui, S. Yang, X. Yan, J. Leng, S. Shuang, P. M. Ajayan and Z. Zhang, Adv. Funct. Mater., 2016, 26, 5708-5717. 57. P. Chen, K. Xu, Y. Tong, X. Li, S. Tao, Z. Fang, W. Chu, X. Wu and C. Wu, Inorg. Chem. Front., 2016, 3, 236-242. 58. J. Wei, Y. Hu, Z. Wu, Y. Liang, S. Leong, B. Kong, X. Zhang, D. Zhao, G. P. Simon and H. Wang, J. Mater. Chem. A, 2015, 3, 16867-16873. 59. L. Lin, Q. Zhu and A.-W. Xu, J. Am. chem. Soc., 2014, 136, 1102711033. 60. Y. J. Sa, D.-J. Seo, J. Woo, J. T. Lim, J. Y. Cheon, S. Y. Yang, J. M. Lee, D. Kang, T. J. Shin, H. S. Shin, H. Y. Jeong, C. S. Kim, M. G. Kim, T.-Y. Kim and S. H. Joo, J. Am. chem. Soc., 2016, 138, 15046-15056. 61. G. Panomsuwan, N. Saito and T. Ishizaki, Phys. Chem. Chem. Phys., 2015, 17, 6227-6232. 62. Y. Zhang, S. Chen, Y. Wang, W. Ding, R. Wu, L. Li, X. Qi and Z. Wei, J. Power Sources, 2015, 273, 62-69. 63. Y. Li, W. Zhou, H. Wang, L. Xie, Y. Liang, F. Wei, J.-C. Idrobo, S. J. Pennycook and H. Dai, Nat Nano, 2012, 7, 394-400.

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High-temperature annealing of trimetallic ZIF facilitates the formation of carbon nanotube interpenetrated porous doped carbon alloy, displays excellent oxygen reduction activity in alkaline medium.

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