Graphitic nanostructures in porous carbon framework significantly enhance the electrocatalytic oxygen evolution

A hybrid structure, a graphitic nanostructures@porous carbon framework, is developed by utilizing the bimetallic zeolitic imidazolate framework-8 (ZIF-8) as a solid precursor, simultaneously templating porous carbon and growing graphitic nanocarbon in a simplified chemical vapor deposition (CVD) fashion. A ligand, 2-methylimidazolate (2MIM), in the ZIF-8 decomposes above 600 °C to yield active carbon/hydrocarbon radicals/vapour. With the idea of using the high catalytic activity of nickel to grow graphitic nanostructures in a CVD process from gaseous carbon feedstocks, a precursor, bimetallic ZIF-8, is synthesized by partial substitution of zinc metal centres by nickel. Such nickel centres thus act as nanocatalysts to grow graphitic nanostructures from the carbon radicals arising from the partly decomposed ligand of the framework during the carbonization step. These hybrid structures show a highly enhanced electrocatalytic activity for the water splitting oxygen evolution reaction (OER). Furthermore the catalytic activity for the oxygen reduction and hydrogen evolution reactions (ORR and HER), and gas uptake capacities for H2 and CO2 are enhanced with respect to the increased porosity and nitrogen doping in the samples. We also show that not all the MOF-based precursors with nickel metal centres are suitable for producing nanographitic structures. Our further investigation suggests that the graphitization in the samples plays a critical role in enhancing the catalytic activities.

*Corresponding author: gsrinivasphys@gmail.com; s.gadipelli@ucl.ac.uk Figure S1.Synthesis of ZIF-8 and bimetallic Zn/Ni-ZIF-8 and their carbon nanostructure hybrids: The methanol solution stirring method at room temperature resulted only formation of Zn-ZIF-8. 5The hydrothermal method yielded Zn/Ni-ZIF-8 from N,N-dimethylformamide solution of precursors. 44The carbonization is carried out under inert gas, N 2 flow in a horizontal furnace.
Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is © The Royal Society of Chemistry 2017           The enhanced ORR and HER activity can be understood either from increased current density at more positive potential, and also with a more positive on-set reaction potential.It was found that nickel nanocentres and its induced severe graphitization with a considerable loss of nitrogen functionality to reduce the heterogeneous surface and positive cores for effective oxygen adsorption in the structures eventually leads to unfavourable ORR activity (see reference 5 in the main text).The bifunctional activity of the samples for ORR and OER is summarized in table (d).Note that the potential is estimated from the difference between the OER (at 10 mA cm −2 ) and ORR (at −3 mA cm −2 ), and lower the difference better the bifunctional activity.Accordingly, ZNDC1100 and ZNDC1000 samples show better bifunctional activity compared to the ZDC1000, in its amorphous state.ZDC1000 sample require additional potential of about 320 mV to that of ZNDC1000 sample.

Figure S4 .
Figure S4.SEM micrographs of ZNDC1100 sample.A melting behaviour and onset growth of nanographites@porous carbon structure is seen in the sample heated to the target temperature 1100 °C and cooled down immediately (a).Surface gold coated nanoparticles are seen in the images taken at high magnifications (c & e).

Figure
Figure S5.TEM micrographs of ZNDC1000 sample, showing a coexistence & growth of nanographitic structures in the amorphous type porous carbon.

Figure
Figure S6.TEM micrographs of ZNDC1100 sample, showing a coexistence & growth of nanographitic structures in the amorphous type porous carbon.

Figure S7 .
Figure S7.SEM micrographs of ZDC1000 derived from Zn-ZIF-8, shows only micro-cracks and surface shrinkage due to the evaporation of zinc metal together with the partial decomposition of the ligand.

Figure S8 .
Figure S8.XPS spectra of N 1s (top-left), O 1s (top-right), C 1s (middle-left), Zn 2p (middleright), and Ni 2p (bottom-left).Table shows the elemental composition in atom%.All the intensities showed are normalized to C 1s peak intensity.Same colour labels apply for all the plots.Framework and ligand decomposition can be understood from the N 1s and O 1s spectra.Oxygen is detected in the structure due to the atmosphere adsorbed moisture on the defective sites.Therefore, ZNDC700 sample shows highest oxygen content on high concentrated defect sites at C, N and Zn, generated by framework decomposition.Increase in the carbonization temperature leads to the transition from pyrrole N-type in the ZIF-8 to the pyridine N-and graphitic N-type.Increased carbonization temperature leads to the graphitization by loss of nitrogen.Complete evaporation of Zn metal happens at >900 °C.The reduced Ni content in the ZNDC1000 and ZNDC1100 compared to the ZNDC900 is the indication of graphitization at Ni centres leading to the shielding effect.

Figure S9 .
Figure S9.Porosity characteristics of the ZNDCs: Pore size distribution (top-left), cumulative pore volume (top-right) curves.Middle panel shows the N 2 adsorption-desorption isotherm and its QSDFT derived pore-size distribution for the ZNDC700 sample.Bottom-left plot shows the relation between QSDFT model derived total micropore volume at 2 nm and BET specific surface area (SSA) of the ZNDC and ZDC samples.Due to the framework decomposition, the ZNDC700 sample exhibits high microporosity.Table lists the BET SSA and total pore volume (V t ) of the samples.

Figure S10 .
Figure S10.Structure and porosity induced gas uptake behaviour of ZNDCs: CO 2 uptake at 298 K (top-left) and H 2 uptake at 77 K (top-right) isotherms.Bottom two plots show the CO 2 and H 2 uptake at 1 bar against the BET surface area of the samples.High gas uptake in the ZNDC700 sample is directly attributed to the structure defects of active C, N and Zn cites and microporosity due to the framework decomposition.

Figure
Figure S11.TGA curves of ZDC1000 and ZNDC1100 samples, measured under oxygen atmosphere with a heating rate of 5 °C per minute.See that different thermal stability of the samples is directly attributed to the amorphous and graphitized carbons.The two-step massloss in the ZNDC1100 sample indicates that porous carbon and nanographites in about 1:1 weight ratio.Complete burning of the carbon estimates about 11 mass% of nickel in the structure, calculated assuming Ni is in NiO state under high temperature and oxygen atmospheric conditions.

Figure S12 .
Figure S12.CV curves for ORR (a) and polarization curves for ORR (b) and HER (c) of the ZNDC samples, measured under 0.1 M KOH electrolyte at a scan rate of 10 mV s −1 .ORR activity can be seen at development of more promising cathodic current peak at about 0.75 V (a).The enhanced ORR and HER activity can be understood either from increased current density at more positive potential, and also with a more positive on-set reaction potential.It was found that nickel nanocentres and its induced severe graphitization with a considerable loss of nitrogen functionality to reduce the heterogeneous surface and positive cores for effective oxygen adsorption in the structures eventually leads to unfavourable ORR activity (see reference 5 in the main text).The bifunctional activity of the samples for ORR and OER is summarized in table (d).Note that the potential is estimated from the difference between the OER (at 10 mA cm −2 ) and ORR (at −3 mA cm −2 ), and lower the difference better the bifunctional activity.Accordingly, ZNDC1100 and ZNDC1000 samples show better bifunctional activity compared to the ZDC1000, in its amorphous state.ZDC1000 sample require additional potential of about 320 mV to that of ZNDC1000 sample.

Figure S13 .
Figure S13.Structural characteristics of ZNDC1100-ox, sample obtained by controlled oxidation of ZNDC1100 sample.PXRD patterns show the partial surface oxidation of nickel to yield NiO@ Ni@C (a), N 2 adsorption-desorption isotherms of ZNDC1100 and ZNDC1100-ox samples reveal that reduced porosity in the oxidized samples, directly attributed to the partial burning of the porous carbon (b), XPS O 1s (c) and Ni 2p (d) spectra shows the formation of surface NiO@Ni in the ZNDC1100-ox sample.

Figure S14 .
Figure S14.ORR polarization curves at 1600 rpm in a 0.1 M KOH electrolyte for the ZNDC1100 and ZNDC1100-ox samples, showing a reduced activity of the ZNDC1100-ox.

Figure
Figure S15.TG curves of ZNDC1100 and ZNDC1100-ox samples, measured under oxygen atmosphere with a heating rate of 5 °C per minute.See that different thermal stability of the samples is directly attributed to the amorphous and graphitized carbons.ZNDC1100-ox sample exhibits predominantly graphitized carbon.Complete burning of the carbon estimates about 35 mass% of Ni content in the ZNDC1100-ox sample, which is a three times higher than ~11 mass% Ni in the ZNDC1100 sample.

Figure S16 .
Figure S16.Characteristics of Ni-MOF-74 and derived carbons -as-synthesized and after acid etching of nickel.Top panel shows the PXRD patterns and N 2 adsorption-desorption isotherms (at 77 K) of the Ni-MOF-74 and MDC1000, and the magnetic behaviour and SEM micrographs of MDC1000.Bottom panel shows comparative N 2 adsorption-desorption isotherms of MDC1000 sample before and after acid washing, see the Ni 2+ ions during the nickel etching under acid, and SEM micrographs of the MDC1000-aw.The samples exhibit BET SSA and total pore volume of (800, 150 & 710) m 2 g −1 and (0.34, 0.16 & 1.03) cm 3 g −1 , respectively for Ni-MOF-74, MDC1000 & MDC1000-aw.

Figure
Figure S17.TEM micrographs of MDC1000-aw, showing predominant porous carbon with the regions of graphitic shells formed around nickel clusters (etched by HCl washing).

Figure S18 .
Figure S18.Comparative OER activity curves of the samples show a significantly enhanced catalytic activity in the hybrid ZNDC samples over the simple nitrogen doped porous ZDC1000 sample or dopant free MDC1000-aw sample.A benchmark reference sample, IrO 2 /C [ref.32] is also included to show our sample, ZNDC1100 superior activity.

Figure S19 .
Figure S19.Comparative OER data curves of our ZNDC1100 with the literature samples of metal/alloy-doped (N-, S, P-)-carbon structures, include graphene-, nanotube-and porous carbon-based network substrates.Reference IrO 2 /C sample is also included as a standard.All the data were recorded in a 0.1 M KOH electrolyte at 1600 rpm of the rotating disk electrode (see the references in the main paper for full details).

Figure
Figure S20.Comparative OER data curves of the literature samples of pristine/ or oxidized carbon structures, include graphene (G or reduced graphene oxide, rGO)-, nanotube (CNT, or MWCNT)-and porous carbon (HMSC-hollow mesoporous shell carbon, OMC-ordered mesoporous carbon, C cloth-carbon cloth)-based network substrates.All the data were recorded in a 0.1 M KOH electrolyte (see the list of additional references for full details).

Figure S21 .
Figure S21.Comparative OER data curves of the literature samples of metal-free, N-, P-, Sdoped carbon structures, include graphene (G)-, nanotube/nanofiber (CNT/CNF)-and porous carbon (HMSC-hollow mesoporous shell carbon, MC-mesoporous carbon, C-porous carbon, CNS-carbon nitride sponge)-based network substrates.All the data were recorded in a 0.1 M KOH electrolyte (see the list of additional references for full details).

Figure S22 .
Figure S22.Comparative OER data curves of the literature samples of metal / alloy / metaloxides @ doped carbon structures, include graphene (rGO, or G)-, nanotube (CNT, or MWCNT)-and porous carbon (HMSC-hollow mesoporous shell carbon, MC-mesoporous carbon, C-porous carbon)-based network substrates.All the data were recorded in a 0.1 M KOH electrolyte.(see the list of additional references for full details).