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Interconnected Copper Cobaltite Nanochains as Efficient Electrocatalysts for Water Oxidation in Alkaline Medium Ayon Karmakar, and Suneel Kumar Srivastava ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 10, 2017

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ACS Applied Materials & Interfaces

Interconnected Copper Cobaltite Nanochains as Efficient Electrocatalysts for Water Oxidation in Alkaline Medium Ayon Karmakar and Suneel Kumar Srivastava* Inorganic Materials and Nanocomposite Laboratory, Department of Chemistry, Indian Institute of Technology, Kharagpur – 721302, India.

KEYWORDS: Copper cobaltite, Solvothermal, Temperature controlled, Interconnected nanochains, Oxygen evolution reaction, Overpotential

ACS Paragon Plus Environment

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ABSTRACT: The present work is focused on protective agent free synthesis of interconnected copper cobaltite (Cu0.3Co2.7O4) nanochains by temperature controlled solvothermal method followed by post thermal treatment of the precursors. Further, Cu0.3Co2.7O4 interconnected nanochains are employed as electrocatalyst for water oxidation in alkaline medium for the first time. Extensive studies of physio-chemical properties showed the formation of interconnected 1D nanochains of Cu0.3Co2.7O4 exhibiting larger specific surface area (139.5 m2 g-1) and enhanced electrochemical water oxidation ability. It delivered excellent mass activity (~50.0 A g1

), high anodic current density (~124.9 mA cm-2 at 1.75 V vs. RHE) and turnover frequency (~

4.26×10-2 s-1) in 1.0 M KOH. These Cu0.3Co2.7O4 nanochains also demonstrated low overpotential (~351 mV) and good cycling stability (1000 cycles) in strong alkaline media. The fabricated Cu0.3Co2.7O4 nanochains could be a good alternative to the commercial OER electrocatalysts (RuO2, IrO2) and also advantageous to the development of efficient, cost effective and durable electrocatalysts for electrochemical water splitting.

INTRODUCTION Energy is the most important issue of the 21st century faced globally due to unsustainable nature of non-renewable energy resources. In addition, problems are also faced in terms of environmental compatibility due to CO2 emission leading to global warming.1-3 Therefore, several other alternative environment friendly renewable energy sources, such as solar cells, wind energy, hydropower etc. have been harnessed.3 However, unpredictable nature of renewable electricity sources and storage as well as proper distribution of energy remain other key issues associated with this.2 Therefore, considerable amount of research has been focused in recent years on the development of materials from this point of view.


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Electrocatalysis finds an important application in conversion and storage of energy.1 However, targeted efficiency of water electrolysis is still a serious challenge predominantly due to the limitations in the performance of oxygen evolution reaction (OER). This reaction involves four electron pathway both in acidic as well as in alkaline conditions and associated with many energy conversion and storage devices, such as water electrolyser,4 metal air batteries5 and solar fuel production etc.1,4 The oxygen evolution reaction exhibits unusually low rate constants, high overpotential and induces structural changes into electrode materials. As a result, sluggish kinetics, drop in performance and ultimately electrode breakdown accounts for poor efficiency of water splitting.6 Therefore, considerable amount of research work has been focused on development of effective electrocatalysts in oxygen evolution reaction including its emphasis on kinetics and mechanism. Rare-earth oxides, specially, RuO2 and IrO2 are most effective electrocatalysts in OER both in acidic and alkaline media due to their low overpotential and high current densities.7 But their high cost and poor stability in alkaline media for long term, remain most important drawback for their large scale applications.6,8 Therefore, several other alternative cost-effective and earth abundant materials, such as transition metal oxides,9-11 layer double hydroxides12,13 and perovskites14,15 have been investigated as electrocatalysts in OER. In this context, cobalt based materials have been currently gaining much attention as efficient water oxidation catalysts owing to their low cost, easy availability and environment friendliness.16 In this regard, cobalt containing phosphates,17,18 perovskites,15 layered hydroxides19/double hydoxides,20 and metal organic frameworks21 demonstrated promising OER activity. Additionally, plenty of reports are also available on spinel cobaltites as effective electrocatalysts in water oxidation.11,22,23 In view of this, binary spinel cobaltites, such as, MxCo3-


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xO4,

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[M = Ni+2, Cu+2, Mn+2, Zn+2] are attractive electrocatalysts in water oxidation due to low

electrical resistance, optimized compositions and environmental benignity.24-26 Among these, copper substituted cobaltites, CuxCo3-xO4 have been receiving much interests as active OER catalysts owing to their more number of available active sites and multiple valence oxidation states.27-30 CuxCo3-xO4 (x= 0 to 1) prepared through a sol–gel method exhibited high electrocatalytic activity in OER with current density of 1 A cm-2, low overpotential (0.30–0.35 V) at 70 °C in 30% KOH.31 Rosa-Toro32 reported that octahedral sites are preferred by interstitial copper in the Co3O4 lattice influencing the electrocatalytic activity. Chi et al. concluded that OER activity of CuxCo3-xO4 electrodes increases in alkaline media due to tetrahedral occupancy of Co+3 ions.33 Chen et al. noted exceptionally long term stability and large decrease in electrical resistivity in Cu0.3Co2.7O4 electrodes.34 Recently, CuCo2O4 nanoparticles embedded on a nitrogen doped reduced graphene oxide exhibited good OER activity with low overpotential of 0.36 V in 1.0 M KOH, though, current density not so high.30 The available literature also suggest that CuxCo3-xO4 (0 < x ≤ 1) electrodes used in OER were mostly films or coatings on conductive substrates. However, there exist very limited amount works on nanostructured CuxCo3xO4

electrodes in absence of any conducting support. Recently, fabrication of morphology

oriented nanostructures exhibited enhanced electrocatalytic performance.22,35,36 These studies revealed that 1D nanostructures could act as an efficient electrocatalyst owing to their high aspect ratios, porous nature, low electrochemical charge transfer resistance and short mass transfer or diffusion distances. Porous Co3O4 nanochains obtained by decomposition of cetyl-trimethyl ammonium bromide (CTAB) assisted oxalate precursor, act as efficient bi-functional electrocatalyst towards O2-electrocatalysis in alkaline medium.22 Manganese-cobalt oxide (MnCo2O4 and CoMn2O4) nanofibres fabricated via a electrospinning technique showed bi-


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functional behaviour for rechargeable metal-air batteries.35 NixCo3-xO4 nanowire arrays grown on conducting titanium (Ti) substrates via ammonia-evaporation-induced growth exhibited good electrocatalytic OER performance.36 However, investigations pertaining to OER in the interconnected 1D transition metal oxides are still lacking. It is anticipated that presence of such interconnecting network could further facilitate electronic transport and increase contact area between the active 1D material and electrolyte.37 Fascinated by these, we have focused our work on syntheses of interconnected 1D nanochains of Cu substituted Co3O4, i.e., CuxCo3-xO4 (0 < x < 1) by simple solvothermal method using ethylene glycol as a chelating solvent and characterized. The fabricated interconnected 1D nanochains of Cu0.3Co2.7O4 spinel have also been examined as electrocatalysts in OER. Our findings showed that interconnected Cu0.3Co2.7O4 nanochains of high specific surface area exhibited enhanced electrocatalytic performance in OER. EXPERIMENTAL SECTION Materials. Copper nitrate trihydrate [Cu(NO3)2.3H2O], cobalt nitrate hexahydrate [Co(NO3)2.6H2O], ethylene glycol [C2H6O2], urea [CO(NH2)2], potassium hydroxide [KOH], and nafion (5 wt%) were procured from S.D. Fine Chemicals, Loba Chemicals Mumbai, India, NICE Chemicals, Merck India, Universal Laborataries, Mumbai, India and Sigma-Aldrich respectively. All these reagents were of analytical grades and used without further purification. Preparation of Copper Cobaltite (CuxCo3-xO4) nanostructures. In a typical procedure, 4 mmol Cu(NO3)2.3H2O and 8 mmol Co(NO3)2.6H2O were dissolved in 60 mL of ethylene glycol by constant stirring for few minutes. Then 20 mmol CO(NH2)2 was added to the earlier mixture and stirred for 30 minutes. Subsequently, entire contents were


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transferred to 100 mL teflon lined stainless steel autoclave at 180 °C for 24 hours. The pink colored precipitate formed was filtered and washed with distilled water and ethanol for several times and finally kept in a vacuum oven at 60 °C for overnight. In addition, controlled experiments were also done at 150 °C and 210 °C temperatures keeping all other reaction parameters unchanged. The obtained solvothermal products at three different reaction temperatures were considered as the precursors. The powdered precursors were subjected to calcination at 400 °C for 3 hours in air atmosphere in a muffle furnace. The samples prepared in this manner at 150 °C, 180 °C and 210 °C were marked as CCO-150, CCO-180 and CCO-210 respectively. The detailed schematic representation about fabrication of copper cobaltite nanostructures is also displayed in Scheme 1. For comparison, Co3O4 (sample referred as CO180) was also prepared using 12 mmol Co(NO3)2.6H2O at 180 °C while maintaining conditions identical as earlier. Scheme 1. Synthetic scheme of CCO-150, CCO-180 and CCO-210.


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Characterization Techniques. X-ray diffraction (XRD) study of the synthesized CuxCo3-xO4 were carried out at room temperature on a Bruker D8 Advance with Cu Kα radiation in the scan ranges of 2θ = 10° – 80°. Thermal stability measurements were analyzed in range 30–800 °C at a heating rate of 10 °C min-1 under air atmosphere in a Q50 thermogravimetric analyzer (TA Instruments). FTIR analysis was performed using Perkin-Elmer (FTIR spectrometer RXI) using a KBr disk and thin film over the wave number range 400 – 4000 cm-1. The morphology and composition of the synthesized samples were examined on a Carl-Zeiss MERLIN field emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) was performed on a FEI-TECNAI-G2 at an operating voltage of 200 kV. This instrument was also used to obtain selected area electron diffraction (SAED) and lattice fringe of the samples. Surface areas and pore size distributions of the samples were determined on Micromeritics 3Flex instrument by volumetric nitrogen adsorption/desorption isotherm using Brunauer–Emmett–Teller (BET) formulations and Barrett–Joyner–Halenda (BJH) method respectively. X-ray photoelectron spectroscopy (XPS) of the samples was performed on a PHI 5000 Versa Probe II (ULVAC–PHI, INC, Japan) system using a micro-focused (100 µm, 25 W, 15 kV) monochromatic Al Kα source (hν = 1486.6 eV), a hemispherical analyser, and a multichannel detector. Electrochemical characterization Preparation of working electrode. A rotating disk electrode with glassy carbon disk (GC-RDE) of 3 mm diameter was used for all the electrochemical characterization of the materials. Initially, GC-RDE was polished with 0.05 µm alumina powder and its surface cleaned by sonication in presence of distilled water. For the


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preparation of catalyst ink, 4 mg catalyst was dispersed in 1 mL water-isopropanol mixture (3:1, v/v) containing 10 µL (5 Wt.%) ethanolic solution of nafion through sonication for 1 hour. Subsequently, 3.5 µL of the catalyst ink was dropcasted on GC-RDE and left for drying overnight at room temperature. The mass loading of the prepared catalysts on GC-RDE was ~0.20 mg cm-2. Electrochemical measurements were performed in a standard three electrode cell configuration (saturated calomel electrode (SCE), Pt wire and GC-RDE as reference, counter and working electrode respectively) on a CHI 7086E electrochemical workstation (CH Instruments Inc., USA). The freshly prepared 1.0 M KOH solution saturated with O2 by bubbling O2 for 30 minutes was used in all electrochemical measurements. All the measurements were performed with a rotation speed of 1600 rpm to get rid of the produced oxygen bubbles. All the polarization curves were recorded without iR – compensation. Electrocatalytic activity of the CCO-150, CCO-180, CCO-210 and CO-180 in OER were studied in O2 saturated aqueous 1.0 M KOH by following anodic sweep. It is to be noted that all the potentials referred in this work correspond with respect to Reversible hydrogen electrode (RHE). Prior to investigations, stable cyclic voltammograms (CV) were obtained by subjecting it to ~ 30 cycles in the potential range of 1.25 V to 1.85 V. The linear sweep voltammetry (LSV) curves corresponding to OER were recorded in the potential range of 1.45 V–1.75 V vs. RHE at a scan rate of 1 mV s-1. Polarization (LSV) curves were taken at a scan rate of 1 mV s-1 to avoid the capacitive currents during the measurements.30 In addition, electrochemical impedance spectroscopy (EIS) of the prepared samples was studied at 1.65 V in the frequency range of 0.01 Hz – 100 kHz with AC amplitude of 5 mV. In order to understand about OER stability, CV was


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obtained at scan rate of 100 mV s-1 in the potential range of 1.45 V – 1.75 V for 1000 cycles. It was subsequently followed by recording of its polarization curve of these same samples. RHE calibration. The potentials vs. SCE were calibrated with respect to RHE by placing two Pt–wires in high purity H2–saturated electrolyte acting as working and counter electrode respectively.38 Thereafter, LSV was run at 10 mV s-1 scan rate and the potential (Figure S1) at which the current crossed zero was considered as the thermodynamic potential for hydrogen electrode reactions. Accordingly, ERHE and ESCE are interrelated in 1.0 M KOH as: = + 1.048 Overpotential (η) of OER was calculated at 10 mA cm-2 current density (j) based on relationship:30 = − ° / , where, ° / is the thermodynamic potential for water oxidation and equals to 1.23 V (vs. RHE). Tafel plots (η = a + b log j, where b is tafel slope)39 were recorded by plotting η versus log (j) at 1 mV s-1 scan rate and linear portion of the curve was considered to calculate the tafel slope. Mass activity (A g-1) of the prepared catalysts was calculated with the help of catalyst loading, m (0.20 mg cm-2) and measured current density, j (mA cm-2) as follows:40 =

! "

Turnover frequency (TOF) of the samples was obtained assuming every metal atom is electrocatalytically active from the relation:40 #$% =

! . &'() 4 . * . %


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, where, j is the current density, Sgeo (0.07 cm2) is the geometric surface area of the working electrode and the number 4 is used as OER involves 4 electrons per mole of O2. F is the Faraday constant (96,485.3 C mol-1) and n is number of moles of metal atoms on the working electrode. RESULTS AND DISCUSSION Physico-chemical characterization of precursors. The precursors formed under solvothermal reaction at 150, 180 and 210 °C temperatures, were characterized by XRD and corresponding findings are displayed in Figure 1. The absence of any sharp peaks in XRD pattern of the precursor formed at 150 °C indicated its amorphous nature. Further, XRD pattern of the precursor obtained at 180 °C (ICDD No.- 78-0209) showed crystalline nature of rhombohedral phase of the metal carbonates (Figure 1b). Interestingly, XRD of the precursor formed at 210 °C showed presence of highly intense peaks of cubic phase of metallic Cu (ICDD No.- 85-1326) due to reduction of Cu+2 ions by ethylene glycol,41 though, peak(s) of carbonate(s) of the precursor not clear (Figure 1c).

Figure 1. XRD pattern of the solvothermal products obtained at (a) 150, (b) 180 and (c) 210 °C.


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Surface chemistry of the precursors was analyzed by FTIR. The corresponding spectra (Figure S2) have been recorded in order to identify the presence of metal–oxygen, CO32-, C–H and O–H functionalities. The bands appeared within the range of 1500 – 700 cm-1 could be assigned to the vibrations of carbonate (CO32-).42 Bands at around 1478, 1074, 862 and 740 cm-1 may be assigned to the fingerprint peaks of CO32- ion (D3h symmetry) and ascribed to ν3 (E'), ν1 (A1'), ν2 (A1'') and ν4 (E'') respectively.43-45 A broad band with its maxima located at around 3479 cm-1 and an intense band at 1634 cm-1 correspond to the stretching vibration and bending vibrations of O–H group44 in ethylene glycol present as an impurity. However, this band is slightly shifted to lower wave number (3430 cm-1) in precursor prepared at 180 °C. This could be ascribed to the presence of hydrogen bonding between hydroxyl group of ethylene glycol and oxygen present in metal carbonate precursors. The bands appeared at 2926 and 2853 cm-1 in the spectra in Figure S2 could be assigned to asymmetric and symmetric C–H stretching vibrations respectively. Further, presence of peaks at around 550 cm-1 correspond to M–O (M= Cu, Co) stretching vibrations in the precursors.46 Interestingly, additional sharp band at 1582 cm-1 in precursor prepared at 210 °C correspond to C – H bending vibration.47 In addition to the structural features, morphology of the precursors was also investigated by FESEM and findings are displayed in Figure S3 (a, b, c). Small particle type morphology was observed for precursors at 150 °C. Interestingly, precursor formed at 180 °C showed formation of interconnected nanowire bundles that can be explained on the basis of “oriented attachment” theory.48,49 In view of this, particles may get chemically attached together on approaching of two surfaces with similar crystallographic orientation towards one another. The driving force for this type of attachment may be attributed to minimize the energy by merging the high energy facets.49 As a consequence, oriented attachment of the similar surfaces resulted in the formation of


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interconnected nanowire bundles. Further increase in reaction temperature to 210 °C inhibited growth of these interconnected nanowire bundles and formed larger particles of the precursor. However, presence of both the metals (Cu and Co) was observed from the EDX analysis (Figure S3d, e, f) of the precursors at three different reaction temperatures. From the above findings, it can be inferred that the reaction temperature had pronounced effect on the morphology of the precursors. Thermogravimetric (TG) analysis of CCO-150, CCO-180 and CCO-210 precursors were carried out in air atmosphere in the temperature range of 30–800 °C and corresponding findings are displayed in Figure S4. It is noted that order of onset decomposition temperature of the precursors increase in the order: CCO-150 (185 °C) < CCO-180 (200 °C) < CCO-210 (205 °C). Initial weight loss in CCO-150, CCO-180 and CCO-210 precursors in the temperature range of 32–200 °C were found to be 12.2, 7.5 and 11.2 wt. % respectively. This could be ascribed to the expulsion of adsorbed water/ethylene glycol in the precursors.50 The second weight loss in TG in the temperature range of ~200–300 °C refers to the fast degradation of precursors resulting in the formation of spinel oxides from the corresponding precursors as established by XRD earlier. Thereafter, weight loss in TG remained more or less constant in CCO-150, CCO-180 precursors. Interestingly, CCO-210 precursor exhibited a small weight gain (~1.6 wt. %) in the range of 336–419 °C, in all probability due to the oxidation of metallic copper to copper oxide. Based on these studies, we subjected all the precursors for calcination at maintained about ~ 400 °C. Structural analysis of calcined precursors. Figure 2 shows X-ray diffractograms (XRD) of calcined CCO-150, CCO-180 and CCO-210. The peaks appearing at around 2, values of ~19.0, 31.3, 36.9, 38.4, 44.8, 55.6, 59.4, 65.3° in CCO-180 and CCO-150 correspond to respective (111), (220), (311), (222), (400), (422), (511)


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and (440) planes of cubic Cu0.3Co2.7O4 (ICDD No. 25-0270). Further, location of these planes in CCO-210 matched well with the polycrystalline Co3O4 spinel (ICDD No. 80-1535). Interestingly, XRD patterns of CCO-150 and CCO-180 showed absence of any additional peak (s) due to other impurities. In contrast, few additional peaks appeared at ~32.5, 35.5, 38.7, 48.8, 61.5, 66.2 and 68.1° in diffractogram of CCO-210 corresponding to the formation of monoclinic phase of CuO (ICDD No. 02-1040).

Figure 2. XRD patterns of (a) CCO-150 (b) CCO-180 and (c) CCO-210. Surface morphology and microstructure analysis of calcined precursors. Surface morphology of the calcined products was characterized by FESEM and images are displayed in Figure 3a, b and c respectively. It is noted that calcined CCO-180 precursor exhibited formation of interconnected nanoparticles in the form of 1D chain. Further, volume expansion of the product was observed, owing to the porous framework. On the contrary, CCO150 showed formation of aggregated particle like morphology. In case of calcined CCO-210 precursor, presence of Co3O4 nanoparticles along with CuO of octahedral shape as established by EDX (Figure S5) is clearly visible. It may be noted that CuO was formed due to the oxidation of


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copper initially present in the precursor (Figure 1c). The average particle sizes (Figure 3g, h, i) in CCO-150, CCO-180 and CCO-210 precursors were found to be ~21.7, 17.2 and 30.4 nm respectively. TEM of the corresponding samples displayed in Figure 3 (d, e, f) also supported inference based on FESEM.

Figure 3. FESEM images (a, b, c), corresponding TEM images (d, e, f) and particle size distributions (g, h, i) of CCO-150, CCO-180 and CCO-210 respectively. In addition, the interconnected network of nanochains are observed (Figure S6a, b) in CCO180 and the interconnectivity is also clear from the corresponding TEM image (Figure S6c). Additionally, HRTEM image of the sample displayed in Figure S6d, indicated growth of lattice fringes corresponding to (311) and (111) planes (d111= 0.46 nm and d311= 0.24 nm). The SAED


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pattern is shown in inset (Figure S6d). The polycrystalline nature of the sample was indicated by the concentric rings corresponding to (440), (511), (400), (311), (220) and (111) in SAED. Eventually, the elemental composition and distribution in CCO-180 was investigated by EDX (Figure S7b) and elemental mapping (Figure S8) respectively. The EDX spectrum indicated that CCO-18O consists of Cu, Co and O elements and no other impurities were observed. Further, the Cu to Co atomic ratio (~0.10) is in good agreement with Cu/Co atomic ratio (~0.11) of copper cobalt oxide (ICDD No. 25-0270). Thus, it can be inferred that the plausible chemical formula of the prepared CCO-180 is Cu0.3Co2.7O4. Surface elemental analysis of Cu0.3Co2.7O4 nanochains. The surface elemental compositions and the electronic configurations of the constituent atoms were analyzed by XPS. Figure 4a shows survey scan spectra of CCO-180 in the binding energy range of 0 – 1100 eV. The presence of C, Cu, Co and O elements are clearly inevitable from the survey spectra. Figure 4b, c and d display deconvoluted core level individual spectra of Cu 2p, Co 2p and O 1s respectively. It is noted that Cu 2p3/2 could be fitted with two satellite peaks assigned to Cu+2 and two spin-orbit doublets (Figure 4b). The satellite peaks at 942.6 and 940.1 eV in the spectra corresponds to divalent Cu.28 In addition, peaks corresponding to 933.8 and 931.5 eV could be ascribed to the octahedrally coordinated Cu+2 and tetrahedrally coordinated Cu+ respectively.28,30 The appearance of most intense 933.8 eV peak in spin orbit doublet is ascribed to formation of energetically favourable of Cu+2 structure.30,51 Further, Cu+ peak of Cu 2p originated from x-ray induced reduction of Cu+2.30,51,52 Figure 4c shows deconvoluted Co 2p spectra and has been successfully fitted with Co 2p1/2 (794.1 eV) and Co 2p3/2 (779.1 eV). The spin-orbit splitting of 15 eV between the Co 2p1/2 and Co 2p3/2 peaks confirmed presence of the mixed Co+2 and Co+3 ions and established the formation of spinel oxides.28,30 The weak satellite


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peak at 788.8 eV could be assigned to the octahedral occupancy of Co+3 corresponding to the characteristic of spinel structures.51-53 The deconvoluted spectrum of O 1s in Figure 4d consists of two main components. The presence of a strong peak (529.2 eV) with a shoulder is a characteristic of metal-oxygen binding.28,53,54 This is attributed to the lattice oxygen present in copper-cobalt mixed oxides.28,51 Another peak (broad) appeared at ~530.8 eV in O 1s spectra possibly due to the chemisorbed oxygen28,53,54 or dissociation of metal carbonates to oxides.52

Figure 4. XPS spectra, (a) survey scan, deconvoluted (b) Cu 2p, (c) Co 2p and (d) O 1s of CCO180.


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Surface area and pore size distributions of calcined samples. Surface area measurements and pore size distributions of the CCO-150, CCO-180, CCO-210 and CO-180 samples were performed by N2 adsorption–desorption isotherm method. Accordingly, BET isotherm and BJH model were used to calculate surface area, pore size and pore volume distributions respectively. The corresponding data of all these spinel oxides are displayed in Table 1. N2 adsorption – desorption isotherms and pore size distribution plots shown in Figure S9. The mesoporous nature of the spinel oxides samples is inevitable from the observed hysteresis loop characteristics of Type-IV isotherm. BET specific surface area of the sample follows the order: CCO-180 (139.5 m2 g-1) > CCO-150 (108.3 m2 g-1) > CO-180 (44.45 m2 g-1) > CCO-210 (26.57 m2 g-1). The pore size distribution of CCO-150, CCO-180, CCO-210 and CO180 showed a predominant peak in the range of 9–48, 9–24, 15–48 and 9–47 nm respectively. The corresponding values of average pore diameter are found to be ~21.1, 12.8, 15.2 and 22.0 nm. All these findings suggest that highest surface area and lowest average pore diameter of CCO-180 could be ascribed to formation of interconnected nanochains leading to porous framework. The cumulative pore volume from the desorption isotherm of CCO-150, CCO-180, CCO-210 and CO-180 are found to be 0.601, 0.471, 0.101 and 0.269 cm3 g-1 respectively. Table 1. BET specific surface area, BJH pore size and pore volume of CCO-150, CCO-180, CCO-210 and CO-180 respectively. Sample Codes BET SSA (m2 g-1) BJH pore size (nm) BJH pore volume (cm3 g-1) CCO-150

108.3

21.1

0.601

CCO-180

139.5

12.8

0.471

CCO-210

26.57

15.2

0.101

CO-180

44.45

22.0

0.269


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Electrocatalytic oxygen evolution activity. Electrocatalytic activity of CCO-150, CCO-180, CCO-210 and CO-180 has been evaluated for oxygen evolution reaction. Figure 5a depicts polarization curves for OER at scan rate of 1 mV s1

. The CCO-150, CCO-180, CCO-210 and CO-180 samples exhibit onset potentials at 1 mA cm-

2

current density55 of ~ 1.526, 1.504, 1.567 and 1.597 V respectively (inset of Figure 5a). The

lowest onset potential of CCO-180 indicated its highest OER activity compared to other electrode materials. Further, current density at ~ 1.75 V follows the order: CCO-180 (124.9 mA cm-2) > CCO-150 (118.1 mA cm-2) >> CCO-210 (57.85 mA cm-2). Our study also showed that CO-180 exhibited current density of ~65.57 mA cm-2 and this value is closer to CCO-210 (57.85 mA cm-2). The current density of 124.9 mA cm-2 (~1.75 V vs. RHE) exhibited by CCO-180 is found to be relatively higher than other reported values, such as ~70 mA cm-2 (0.6 V vs. SCE) Co3O4/NrmGO,38 ~65 mA cm-2 (0.7 V vs. SCE) CuCo2O4/NrGO.30 It is anticipated that unique 1D interconnected chain like morphology of CCO-180 accounts for its maximum OER activity due to its highest surface area as well as presence of large number of channels facilitating diffusion of OH– ions of the electrolyte. The OER activity of CCO-150, CCO-180, CCO-210 and CO-180 catalysts were compared at the current density of 10 mA cm-2 in relevance to hydrogen fuel synthesis.56 It is noted that CCO180 attains 10 mA cm-2 current density at overpotential (η10) of ~351 mV. In case of CCO-150, CCO-210 and CO-180 this current density was achieved at ~375, 418 and 420 mV overpotential respectively (Figure S10). Incorporation of Cu+2 ions in Co3O4 lattice led to decrease the overpotential of CCO-150 and CCO-180 by 45 and 69 mV compared to Co3O4. In contrast, overpotential decrease in CCO-210 was observed only by 2 mV, due to formation of larger particles of Co3O4 along with CuO nanostructures. These findings demonstrate clearly that initial


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preparative temperature of precursors makes pronounced effect on the product morphology and in the OER activity. In addition, inference on effect of Cu/Co ratio (Figure S7) in OER activity of the prepared catalysts has also been evaluated. These study showed inferior OER activity of CCO-150 (Cu/Co=0.04) in contrast to highest OER activity achieved in CCO-180 (Cu/Co=0.10). However, CCO-210 (Cu/Co=0.46) is found to be detrimental in its electrocatalytic activity. The performance of CCO-180 with its optimized composition was also reflected in our subsequent findings on OER. Deng et al. also reported that CuxCoyO4 (Cu/Co=0.12) showed low overpotential and good oxygen evolution ability amongst all other samples having ratio in range of ~0.06 – 0.5.25 Further, overpotential of CCO-180 is competitive to the commercial state-ofthe-art RuO2 (387±0.02 mV),57 and IrO2 (387±0.01 mV)57 and also outperforming other reported CuxCo3-xO4 and doped Co3O4 based electrode materials, such as CuCo2O4/NrGO (360 mV),30 Cu0.7Co2.3O4 (480 mV@ 7 mA cm-2),29 NiCo2O4 (419 mV),58 ZnCo2O4

(390 mV),59

Au/NiCo2O4 (370 mV)60 (Table S2). OER kinetics of CCO-150, CCO-180, CCO-210 and CO-180 were investigated on the basis of Tafel plots and findings are displayed in Figure 5b. The magnitudes of Tafel slope of the corresponding electrocatalysts were found to be ~71.7, 63.3, 77.7 and 64.1 mV dec1

respectively. It is also inferred that CCO-180 exhibited the lowest Tafel slope among the four

prepared electrocatalysts indicating its most efficient OER activity. Further, the Tafel slope is found to be relatively lower than that reported on most of the Co based oxide electrocatalysts, such as CuCo2O4/NrGO (64 mV dec-1),30 NixCo3-xO4 (66 mV dec-1),61 crumpled graphene-CoO (71 mV dec-1),23 Co3O4/N-rmGO (67 mV dec-1),38 NiCo2O4 NWs (90 mV dec-1)62 (Table S2). The Tafel slope close to 60 mV dec-1 indicates the involvement of Co+4 ions as the active species for electrocatalysis.63 In view of this, the Tafel slope of CCO-180 (~63.3 mV dec-1) indicated


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Page 20 of 35

that Co+4 ions act as the active sites for efficient water oxidation.30,64 The critical contribution of Co+4 ions in OER has been extensively discussed by earlier works.40,63 According to the “dband” theory, metal d-states near the Fermi level also account for enhanced catalytic activity.40 It is assumed that electrophilicity of adsorbed oxygen increases in case of higher valent Cocations.40 As a result, the formation of hydroperoxy (OOH) species, as well as its conversion and evolution of O2 is also accelerated.40,63 OER activity is observed in transition metal oxides due to the interaction between oxygen and metal d states. In all probability, σ-bonding between the surface adsorbed O2 molecules and the eg orbital of transition metal ions regulates the population of d-electrons in eg orbitals. Further, bond strength of OOH (intermediate) on catalyst surface is controlled by the electron filling in eg level.40 In contrast, involvement of Co+4 is somewhat reduced in case of CCO-210 accounting for its inferior catalytic activity in OER. It is anticipated that OER follows mechanism in agreement with earlier reports as below:30,39,56 & + $- . → & − $- + 0 . & − $- + $- . → & − $ + -1 $ + 0 . & − $ + $- . → & − $$- + 0 . & − $$- + $- . → & − $1 + 0 . + -1 $ & − $1 → & + $1 [where, S is the active species] Electrochemical impedance spectroscopy (EIS) was employed to investigate the charge transfer resistance (Rct) of the prepared catalysts. Figure 5c, shows the Nyquist plots at 1.65 V vs. RHE in the frequency range of 0.01 Hz – 100 kHz. The corresponding data was fitted to the equivalent circuit as shown at the inset of Figure 5c. The equivalent circuit consists of solution resistance (Rs), intrinsic resistance of the catalyst (R1), constant phase element (CPE1) for oxide


20

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catalyst, charge transfer resistance (Rct) and constant phase element (CPE2) corresponding to the catalyst–electrolyte interface during OER.65 It was observed that two semicircles were obtained for each of the catalysts. The small semicircle appeared in the high frequency region may be assigned to the internal resistance of the catalysts66 and the diameter of this semicircles didn’t show much difference for all four catalysts. In addition, the larger semicircle in the low frequency region can be assigned to the Rct for oxygen evolution at the catalyst – electrolyte interface.65,66 The Rct values obtained from the fitted curves were found to be ~14, 10, 47 and 21 Ω in case of CCO-150, CCO-180, CCO-210 and CO-180 respectively. The lowest charge transfer resistance in CCO-180 is a clear indication of relatively fast electron transport during OER. It is also noted that incorporation of Cu in Co3O4, reduces charge transfer resistance due to its conducting property. It is to be noted that Rct value of CCO-210 is highest than that of other prepared spinel catalysts in all probability due to the formation of highly resistive CuO at elevated reaction temperature (210 °C) along with Co3O4 as evident from our XRD studies. Table 2. OER activity of CCO-150, CCO-180, CCO-210 and CO-180 respectively. Sample Codes

Onset potential (V) vs. RHE

η (mV) at 10 mA cm2

j (mA cm-2) at 1.75 V vs. RHE

Tafel slope Mass activity (A (mV dec-1) g-1) at η = 351 mV

TOF (s-1) at η = 351 mV

CCO-150

1.526

375

118.1

71.7

22.4

0.0191

CCO-180

1.504

351

124.9

63.3

50.0

0.0426

CCO-210

1.567

418

57.85

77.7

7.55

0.0083

CO-180

1.597

420

65.57

64.1

2.30

0.0019


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Page 22 of 35

Figure 5. (a) LSV polarization curves for OER, inset shows onset potentials (at 1 mA cm-2), (b) Tafel plots derived from OER polarization curves, (c) Nyquist plots at 1.65 V vs. RHE, equivalent circuit (inset) and (d) polarization curves before and after 1000 cycles in 1.0 M KOH of CCO-150, CCO-180, CCO-210 and CO-180 respectively. Further assessment of the electrocatalytic activity of the prepared catalysts was performed by measuring the mass activity and TOF, calculated at η = 351 mV and corresponding data is displayed in Table 2. It is clearly evident that CCO-180 exhibited highest mass activity (~50.0 A g-1) among the all four prepared catalysts. The intrinsic OER activity of the catalysts was indicated by TOF considering that all the metal atoms were catalytically active. It is inferred from Table 2 that TOF in CCO-180 is highest (~ 4.26×10-2 s-1) and nearly ~ 2.2, 5.1 and 22.4


22

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times higher than that of CCO-150, CCO-210 and CO-180 respectively. This highest TOF of CCO-180 catalyst further supported our earlier findings on its promising OER activity. In addition to high catalytic activity, stability of the electrocatalysts also remains crucial parameter for long term and practical applicability. Therefore, prepared catalysts were cycled continuously for 1000 cycles in the potential range of 1.45 V to 1.75 V vs. RHE in O2 – saturated 1.0 M KOH solution at 100 mV s-1 scan rate. The polarization curves were taken before and after the stability test (Figure 5d). Subsequent decrease in OER current density at 1.75 V vs. RHE corresponds to ~ 19.9%, 3.3%, 65.9% and 67.6% for CCO-150, CCO-180, CCO-210 and CO180 respectively. This shows that CCO-180 suffered relatively much lesser decrease in current density even after 1000 cycles. All these findings clearly demonstrated that CCO-180 serve as a stable electrocatalyst for water oxidation in alkaline medium. Electrochemical active surface area (ECSA) and roughness factor (RF) of all the prepared CCO-150, CCO-180, CCO-210 and CO-180 catalysts were calculated using the following relationships:30 2& =

34 5

and

6% =

7 89:

, where 2;< , 2= and &'() correspond to double layer capacitance, specific double layer capacitance of an atomically smooth oxide surface and geometric surface area of the rotating disk electrode. In our work, 2= was considered as 40 µF cm-2 for all the catalysts in 1.0 M KOH electrolyte.30 In view of this, charging current (ic) data were obtained at different scan rates (10– 100 mV s-1) for each samples from cyclic voltammetric measurements in the non-faradaic region (0 – 0.1 V vs. SCE) at 0.05 V (Figure S11). Subsequently, 2;< values (Figure S12a) were evaluated for each sample from the slope of the plot of charging currents versus scan rate (ν) as follow:67,68


23


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Page 24 of 35

> = ?2;< ECSA as well as RF data (Figure S12b) provided in Table S1 follow the order: CCO-180 (8.27 cm2) > CCO-150 (6.65 cm2) >> CCO-210 (1.07 cm2) > CO-180 (1.05 cm2). CCO-180 exhibited the highest ECSA and RF values compared to other samples. In all probability, interconnectivity of nanochains is accounting for its short diffusion distances and better exposer to the electrolyte.66 This facilitates penetration of electrolyte through the pores of the catalyst and enhances the electrocatalytic activity of CCO-180 making it efficient electrocatalyst for water oxidation. CONCLUSION In conclusion, we have successfully synthesized interconnected Cu0.3Co2.7O4 nanochains by ethylene glycol mediated solvothermal procedure followed by post heat treatment. It was observed that reaction temperature greatly influences the product morphology and hence OER activity. This Cu0.3Co2.7O4 nanochains showed exceptional anodic current density of 124.9 mA cm-2 and small η = 351 mV to reach 10 mA cm-2 in 1.0 M KOH, which was found to be comparable to the best performing state-of-the-art RuO2 and IrO2 catalysts. Thus it becomes as a cost effective, efficient and non-precious noble metal free electrocatalyst for alkaline water oxidation. Further, this material showed good cycle stability in strong alkaline medium. Our study showed that the synthesized Cu0.3Co2.7O4 interconnected nanochains can serve as low-cost and promising electrocatalyst for the renewable energy technologies, such as water electrolyzers, rechargeable metal-air batteries and viable hydrogen fuel economy.


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ASSOCIATED CONTENT Supporting information Calibration curve of SCE with restpect to RHE, FTIR, FESEM, EDX and TGA of precursors, formation mechanism, EDX spectrum of CuO in CCO-210, FESEM, TEM and HRTEM of CCO-180, EDX spectra of calcined samples, elemental mapping of CCO-180, BET isotherm and pore size distribution of calcined samples, overpotential (η10), cyclic voltammograms, XRD of CO-180, Table and plot of ECSA and RF and a comparative table of OER activity with some other available reports (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Telephone: +91 03222-283334. Author Contributions The work has been done by Mr. A. Karmakar under the supervision of Prof. S. K. Srivastava and equal contributions have been made in writing and review of this manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT S.K.S. is grateful to IIT Kharagpur for providing a grant for the CH Instrument and other necessary facilities in this work. A. K. gratefully acknowledges IIT Kharagpur for providing financial support. A. K. is also grateful to Mr. Sanjeev K. Sharma, Materials Science Centre, IIT Khragpur for his timely help in TGA measurements.


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