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Binder-less, free-standing porous interconnects of Ni-Fe alloy decorated reduced graphene oxide for oxygen evolution reaction Naveen Kumar Chandrasekaran, and Saravanakumar Muthusamy Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02413 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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Binder-less, free standing porous interconnects of Ni-Fe alloy decorated reduced graphene oxide for oxygen evolution reaction Naveen Chandrasekaran,* Saravanakumar Muthusamy CSIR-Central Electrochemical Research Institute, Karaikudi-630006, India. Email: [email protected], [email protected]

Abstract We report the synthesis of light-weight, free-standing Ni-Fe@rGO porous interconnects by carbothermal reduction of Ni-FeOx using graphene oxide (GO) as the reducing agent. Here, we take advantage of the oxygen functionalities present in GO to aid in anchoring the metal ions followed by epoxide-assisted Ni-FeOx@GO network formation. When pyrolyzed under inert conditions, Ni-FeOx@GO networks were converted to Ni-Fe@rGO by simple carbothermal metal reduction at 800oC. The Ni-Fe@rGO monoliths were found to be macroporous, electrically conducting and electrocatalytic towards oxygen evolution reaction (OER). The monoliths exhibited excellent OER activity yielding a current density of 10 mA cm-2 at an overpotential of 350 mV vs. RHE, Tafel slope of 38 mV decade-1 and a TOF value of 50 s-1 on par with the established Ni-Fe based electrocatalysts. Key words: Ni-Fe alloy, graphene oxide, reduced graphene oxide, OER

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Introduction Global energy crisis being a concern, the need for alternate energy sources other than

fossil fuel is imperative. Electrolysis of water to oxygen and hydrogen, reduction of carbon dioxide to useful organics are considered effective in meeting the challenging energy requirements (1-3). In particular, the oxygen evolution reaction (OER) is considered an ideal source of clean energy but suffers from sluggish kinetics mainly due to the large overpotentials required in excess of thermodynamic potential (1.23 V) required for water-splitting. Various catalysts have been designed and employed to make the oxygen generation more efficient (4). Hitherto, precious metal oxides such as RuO2, IrO2 are the benchmark catalysts for OER with overpotentials less than 200 mV at a current density of 10 mA cm-2 (5-6). However, low earthabundance and high costs limit their availability for large scale industrial use. In pursuit of effective alternatives for precious metals, monolithic 3-D interconnects commonly referred as aerogels of transition metal, oxides, and alloy particles have fascinated researchers globally due to their enhanced physico-chemical properties such as high porosity, and specific surface area.Diverse synthetic strategies have been reported for the preparation of metal or alloy aerogels by various groups. For example, Leventis etal. reported the synthesis of porous transition metal aerogels and rare earth carbides by carbothermal reduction (7-9). Tappan etal. synthesized macroporous metal foams by combustion method (10, 11). Eychmüller's group reported ordered macroporous noble (mono-/bi-)metallic aerogels by controlled aggregation process with enhanced electrocatalytic activity (12-14). The metal/alloy porous networks combine the unique properties of the individual metal and aerogels. For example, high porosity offers enhanced mass transfer, high specific surface area more active sites and the metallic or the carbonaceous back bone provides superior electrical conductivity necessary for the effective electrocatalytic activity. For instance, the mixed Ni-Fe


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oxides and layered double hydroxides (LDH’s) exhibit good catalytic activity towards oxygen evolution on par with their precious counterparts (15).The enhanced catalytic activity of Ni-Fe catalysts was earlier traced to the presence of Fe (16). Nevertheless, the poor electrical conductivity of transition metal oxides restricts its usage as effective active electrode material. To overcome this drawback, conducting carbons like graphene, amorphous carbon etc have been introduced (by either mechanical or vapor deposition methods with transition metal oxides) to provide a right balance between the current collection and the active phase thereby effectively enhancing the overall charge transfer kinetics (17). Recently, Wang et al. demonstrated the synthesis of Ni-Fe/C sandwich structures via ion-diffusion exchange process; the procedure involving a time consuming multi-step preparation of Ni-Fe/C. A significant barrier to catalyst immobilization approaches arises from the limited stability of surface binding. Ideally, catalysts can be stabilized by binding them to the support / surface thus enhancing the turn over numbers, as has been demonstrated by Meyer et al for anchoring the ruthenium species to the metal oxide surface for the oxygen evolution reaction (18). In the present case, reduced graphene oxide surface serves as the platform for chemically forming films of Ni-Fe oxides which are converted into monolithic alloys. These may be contrasted with methodologies that depend on the use of binding agents for immobilizing the catalyst layers, for example, Pd aerogels using Nafion as the binder. To the best of our knowledge, excluding the electrodeposited Ni-Fe based films, external polymeric binders or conductive materials have invariably been used to enhance the adhesion and electrical conductivity of the electrode material (19). Keeping this in mind, we have designed a strategy for the surface stabilization of electrocatalysts on inert solid surfaces taking advantage of the rGO chemistry for binding the metal ions, forming the corresponding mixed oxides and converting to monolithic alloys using carbothermal reduction. Higher number of sp2


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domains (graphitization) present in rGO is expected to render the support-catalyst system stable under harsh OER conditions. The strong metal-rGO interactions and effective charge transfer due to the overlap between the sp2 and the dsp orbitals of rGO and the alloy particles are also expected to contribute to the properties like electrocatalytic activity. In this work, the transition metal ions are first anchored to the oxygen functionalities (hydroxyl, epoxides and carboxylic acid) readily available in the GO matrix followed by epoxide-assisted metal oxide network formation. The resulting Ni-FeOx@GO monoliths are pyrolyzed under inert atmosphere to obtain a 3-D alloy monolith of Ni-Fe@rGO which is surface-immobilized in situ during these processing stages. The Ni-Fe@rGO monolith is found to exhibit excellent electrochemical oxygen evolution with an overpotential and Tafel slope of 350 mV@10 mA cm-2 and 38 mV decade-1 respectively. 2. Materials and Methods Iron (III) chloride hexahydrate (Sigma Aldrich), nickel chloride (Nice Chemicals), N,N’-dimethylformamide (DMF) and acetone (SRL) were used without any further purification. Graphite powder (300 mesh, 99%) was obtained from Alfa Aesar. Sodium nitrate (NaNO3), Conc.H2SO4 (98%), KMnO4 and 30% H2O2 were used as received from Sigma-Aldrich. 3. Experimental procedure: Graphene oxide was prepared by modified Hummer’s method as reported elsewhere (20, 21).GO of different concentrations (1.3%, 2.6%, 6.2% and 11.6%) were ultrasonicated in 5 mL of DMF for 3 hours. Ni-FeOx@GO gels were prepared by mixing 1.5 mmoles each of NiCl2.2H2O and FeCl3.6H2O in the GO dispersion followed by the addition of 6 mmoles of epichlorohydrin. The sol was then poured into polypropylene molds. The gelation time was found to be ~ 1 h 35 minutes at room temperature. The gelation time was found to be the same


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for gels prepared with different concentrations of GO. The wet gels were washed with DMF (3 x 8 h) subsequently followed by acetone washes (3 x 8 h) to remove the unreacted precursors. The resultant wet gels were dried in an autoclave using scCO2to obtain Ni-FeOx@GOaerogels. The Ni-FeOx@GO aerogels were pyrolyzed under flowing Ar (90 mL min-1) in a tubular furnace at 800 oC to obtain Ni-Fe@rGO monoliths. The pyrolyzed Ni-FeOx aerogels with 1.3%, 2.6%, 6.2% and 11.6% and without GO are designated as Ni-FeOx@rGO-A, Ni-FeOx@rGO-B, NiFeOx@rGO-C, Ni-Fe@rGO and NiFe2O4 respectively. Infrared spectroscopy experiments were performed on a Bruker Tensor 27 FT-IR spectrometer. X-ray diffraction patterns were recorded using a Bruker D8 Advance X-ray diffractometer with a Cu Kα radiation (λ = 1.5418 Å). Morphological studies were carried out using aField Emission Scanning Electron Microscope (Zeiss supra 55VP). N2 sorption isotherms were obtained using nitrogen adsorption/desorption porosimetry (Quantachrome Autosorb-1 surface area analyser). X-ray photoelectron spectroscopy was performed using SPECS, Phoibos 100 MCD Analyzer with pass energy of 20 eV (Al Kα anode (1486.6 eV)) in ultrahigh vacuum (5 × 10–10 mbar). Skeletal densities were determined using a Helium pycnometer (Micrometrics). Bulk density of the samples was calculated from the physical dimensions of the samples. The values of porosity (Π) of the aerogels were determined from the bulk and skeletal density values according to equation, Π = 100 x [(1/ρb) - (1/ρs)]/ (1/ρb).Cyclic voltammetric experiments were performed on a BASi100 workstation. In order to calculate Faradaic efficiency, RRDE experiments were carried out using a VMP3 multi-channel potentiostat (Biologic Inc), a rotator (MSR), glassy carbon (GC) disk-Pt ring (AFE6R2AUPT), of Pine instruments USA.


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Electrochemical Experiments: A conventional three-electrode cell system with Hg/HgO and Pt foil as the reference and counter electrode were used respectively. The GC rotating ring disk electrode was modified with Ni@rGO, Fe@rGO, rGO and Ni-Fe@rGO and subjected to evaluation of oxygen evolution reaction (OER) activity in 0.1 M KOH. Before modification, the glassy carbon electrode was cleaned with 1, 0.3 and 0.05 µ-sized alumina powder and ultrasonicated for about 5 minutes. Thin films of all four samples were drop-cast separately on the glassy carbon electrode. For this, aliquots of 5µL of these samples were prepared by ultrasonicating a mixture of 5 mg of each sample in 1ml of DMF. No polymeric binders were used for the modification of electrode surface. Cyclic voltammetric responses were recorded in a potential range between 0 to 0.6V vs. Hg/HgO at a scan rate of 50 mVs-1.For electrocatalysis of OER, linear sweep voltammetry (LSV) was performed on all four samples between 0 to 0.8 V vs. Hg/HgO at a scan rate of 5 mV s-1. All the potential values were recorded against Hg/HgO and the oxygen evolution curves were plotted against reversible hydrogen electrode (RHE).

Scheme 1: Schematic representation of Ni–Fe@rGO monolith synthesis.


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Results and Discussion Synthesis of Ni-FeOx@GO monoliths: Scheme 1 illustrates the synthesis of Ni-Fe@rGO aerogels. In order to assess the chemical functionalization of the oxygen functionalities present in GO to the Ni-FeOx network, FTIR spectra were recorded for GO, Ni-FeOx and Ni-FeOx@GO and compared in Figure 1a. FTIR spectra of GO reveals the characteristic C=O, C=C, OH deformation vibration, C–OH stretching vibration, C–O vibration, and broad OH peaks at 1708, 1633, 1370, 1224, 1037 and 3400 cm-1 respectively. The pristine Ni–FeOx aerogels show their characteristic peaks at 3404, 1589, 1110 and 495 cm-1 assigned to broad ν OH, δ H2O, δ OH and ν Ni–O or Fe–O respectively. In addition to characteristic peaks present in Ni-FeOx aerogels, the appearance of C=O, C=C and C-O (epoxy) bands at 1727, 1647 and 1230 cm-1 confirms the presence of GO in Ni-FeOx@GO aerogels. Figure 1b compares the XRD patterns of GO, Ni-FeOx and NiFeOx@GO aerogels. By XRD, the pristine Ni-FeOx aerogels show broad reflection at 26o, a typical characteristic of an amorphous material. The neat GO shows its characteristic reflection at 11.9o (001). As expected, Ni-FeOx@GO displays a reflection at 12o followed by a broad reflection indicating the presence of GO and Ni-FeOx respectively. Figure 2a displays the FESEM image of Ni-FeOx@GO aerogels before pyrolysis. The Ni-FeOx aerogels built on GO shows interconnected macroporous structure with particulate morphology. Figure 2b displays the N2 sorption isotherm of Ni-FeOx@GO. The N2 sorption isotherm Ni-FeOx@GO shows a sudden increase in adsorption and the absence of saturation at higher relative pressures (P/Po > 0.8). They also display slim hysteresis loops and substantial specific volumes at low relative pressures demonstrating the presence of meso- and macropores. The BET surface area and the total pore volume (VT) of Ni-FeOx@GO were found to be 86 m2/g and 0.46 cc/g respectively. The


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%porosity calculated from the skeletal and bulk densities (2.61 and 0.74 g/cc) was determined to be 71.5. Synthesis of Ni-Fe@rGO monoliths: Figure S1 displays the step by step procedure for the synthesis of Ni-Fe@rGO monoliths. Figure S2 shows the SEM images of Ni-FeOx@GO aerogels with different concentrations of NiCl2.2H2O and FeCl3.6H2O displaying particulate morphology. The Ni-FeOx@GO aerogels were pyrolyzed under Ar at 800 oC for 3 h to obtain Ni-Fe@rGO monoliths. Different concentrations of GO were added to optimize and ensure the complete conversion of Ni-FeOx to Ni-Fe alloy. X-ray diffraction results indicate that maximum reduction of Ni-FeOx to Ni-Fe alloy phase was obtained when the concentration of GO was 11.6%. Figures 3A compares the XRD patterns of Ni-FeOx with and without GO pyrolyzed at 800oC under flowing Ar. The Ni-FeOx pyrolyzed at 800 oC shows crystalline reflections at 30.47 (220), 35.84 (311), 37.50 (222), 43.48 (400), 54.05 (422), 57.52 (511) and 63.20o (440) that can be attributed to the formation of spinel NiFe2O4. Whereas, the Ni-FeOx@rGO-A, Ni-FeOx@rGO-B, Ni-FeOx@rGO-C displayed NiFe2O4 reflections in addition to the presence of new reflections at 44.66, 51.98 and 76.42o corresponding to metallic Ni and disappearance of peaks at 37.49o and 43.47o confirming the reduction of NiFe2O4 by graphene oxide. Figure 3B shows the XRD patterns of Ni-FeOx with 11.6% of GO pyrolyzed under flowing Ar in a temperature range between 200oC-800oC. The sample pyrolyzed at 200 oC showed a broad amorphous peak. However, the samples pyrolyzed at 400 oC and 600 oC displayed reflections of NiFe2O4 similar to the neat NiFe2O4 samples with additional metallic Ni phases at 44.36 (111) and 51.82 (220) respectively. Nevertheless, NiFe@rGO sample exhibited Ni-Fe alloy phase with the emergence of new peaks at 43.91 (111), 50.92 (220) and 75.26o (220) matching standard patterns of FeNi3 with cubic structure (ICDD #


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98-008-8171). The crystallite size of the Ni–Fe alloy in the Ni-Fe@rGO monoliths calculated using Scherrer's formula works out to be ∼23 nm. As expected, the XRD patterns of Ni@GO and Fe@GO pyrolyzed at 800 oC (Figure S2) show metallic reflections of Ni and Fe with tiny crystalline reflections of its respective oxides. The XRD patterns acquired by pyrolyzing NiFeOx with GO of different concentrations at different temperatures suggests that NiFe alloy phase can be obtained only with NiFeOx with 11.6 wt% of GO at 800 oC. Figure S4 displays the Raman spectra for the neat GO and Ni-Fe@rGO. The presence of rGO in the Ni-Fe@rGO sample in addition to Ni-Fe alloy was confirmed by its characteristic D and G peaks at 1340 and 1523 cm-1. The ID/IG ratio was found to increase for the rGO present in the Ni-Fe@rGO (1.11) relative to the neat GO (0.88) with increase in the number of defects due to thermal treatment and the presence of Ni-Fe alloy (22).The increase in ID/IG ratio of Ni-Fe@rGO sample also suggests a reduction in the average size of the sp2 domains by the formation of new graphitic domains of smaller size relative to pristine GO (23). The XRD patterns and Raman spectra together confirm the formation of Ni-Fe alloy on rGO. Figure 4(a-c) shows the FE-SEM, HR-TEM images and N2 sorption isotherms of NiFe@rGO respectively. It is clearly evident from FE-SEM images that the Ni-Fe alloy particles are embedded on rGO with particle size in the range of < 200 nm. Consistent with the FE-SEM images, HR-TEM images also confirm the presence of Ni-Fe alloy on rGO surface. In addition, the TEM image shows that the Ni-Fe alloy on rGO exists in the form of cubes and spherical particles. Selected area diffraction (SAED) pattern of Ni-Fe@rGO sample displays diffraction rings corresponding to (111), (002), (022) crystal planes of Ni-Fe alloy consistent with the reflections identified in XRD analysis. Figure 5 (A-C) displays the FE-SEM image and the corresponding EDS maps of Ni-Fe@rGO. The EDS maps of Ni-Fe@rGO clearly reveal that Ni-


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Fe alloy particles are uniformly embedded on carbon sheets. The elemental composition quantified from EDS suggests that Ni-Fe@rGO is composed of 70.84, 9.01, 9.53 and 10.47 wt% of C, O, Fe and Ni respectively. The alloy nature of the Ni-Fe alloy particles was confirmed by the perfect overlap of the elemental distribution (24). The presence of oxygen may be due to the oxygen functionalities from rGO and the oxides formed during the surface oxidation of Ni-Fe alloy particles. Figure 6(A-E) displays the survey and high resolution XPS of C1s, O1s, Ni2p and Fe2P of Ni-Fe@rGO. The XPS study of NiFe@rGO reveals the presence of C, O, Ni and Fe in the complete survey spectra. The high resolution C1s can be deconvoluted to C=C/C-C, C-O, C=O and C-(O)-OH centered at 284.89, 286.6, 288.9 and 290.84 eV respectively. The O1s spectrum was deconvoluted into two peaks at 530.2 and 532.0 eV corresponding to C=O and C-O respectively. The presence of C=O and C-O peaks deconvoluted from C1s and O1s spectrum suggests the presence of oxygen functionalities associated with rGO (24). The N1s spectrum contains two peaks at 853.51 and 871.52 eV confirming the metallic state of Ni in NiFe@rGO. A satellite peak of Ni can be found at 860.03 eV. The peaks centered at 707.11 and 721.03 eV in the Fe2p XPS spectrum displayed the metallic state of Fe and the peak at 709.45 eV can be ascribed to the satellite peak of Fe or the oxide formation on the surface of Ni-Fe (24). The physical properties of NiFe2O4, Ni-FeOx-rGO-A, Ni-FeOx-rGO-B, Ni-FeOx-rGO-B, Ni-FeOx-rGO-C and Ni-Fe@rGO samples are shown in Table 1. The bulk and skeletal densities of the samples were used to determine the porosity and for all the samples, the porosity was found to be 72-76%. The BET surface area values for all the pyrolyzed (18-29 m2 g-1) samples were found to decrease relative to the unpyrolyzed sample due to an increase in the particle size as evident from the SEM images displayed in Figure S5.


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Table 1. Physical properties of NiFe2O4, Ni-FeOx-rGO-A, Ni-FeOx-rGO-B, Ni-FeOx-rGO-B, Ni-FeOx-rGO-C and Ni-Fe@rGO

The N2 sorption isotherm of Ni-Fe@rGO shows a rapid increase with no hysteresis at higher relative pressures (P/Po>0.8). Additionally, the adsorption at relatively low relative pressures indicates the presence of micropores. The pore size distribution determined from the BJH method shows the presence of micropores (

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