Facile Synthesis of Self-Assembled Cobalt Oxide Supported on Iron

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Facile Synthesis of Self-Assembled Cobalt Oxide Supported on Iron Oxide as the Novel Electrocatalyst for Enhanced Electrochemical Water Electrolysis Kai Ling Ng, Kuan Ying Kok, and Boon Hoong Ong ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00250 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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ACS Applied Nano Materials

Facile Synthesis of Self-Assembled Cobalt Oxide Supported on Iron Oxide as the Novel Electrocatalyst for Enhanced Electrochemical Water Electrolysis Kai-Ling Ng*, Kuan-Ying Kok# and Boon-Hoong Ong* *Nanotechnology & Catalysis Research Centre, Level 3, Block A, Institute of Graduate Studies, University of Malaya, 50603 Kuala Lumpur, Malaysia. #Malaysian Nuclear Agency, Bangi, 43000 Kajang, Selangor Darul Ehsan, Malaysia.

ABSTRACT: We report, for the first time, a facile, scalable and cost-effective method for the synthesis of highperformance and monodispersed hexagonally-shaped cobalt oxide platelets supported on iron oxides (Fe3O4 and α-Fe2O3) as the electrocatalyst systems for water electrolysis. The Fe3O4 was synthesized in the absence of an inert environment and organic solvent, using a modified co-precipitation procedure. Fe3O4 nanoparticles of average size 15 nm served as the best catalyst support for Co3O4, in Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER). A current density of 10 mA/cm2 was achieved, at -0.36 V and 1.64 V for HER and OER (vs. Reversible Hydrogen Electrode (RHE)) respectively for the Co3O4/ Fe3O4 system, in 0.1 M KOH with low total catalyst loading of 250 µg/cm2 (or 100 µgCo3O4/cm2). Compared to using the same total loading of unsupported Co3O4 catalyst, the overpotentials of Co3O4/Fe3O4 catalyst decreased by 0.22 V and 0.06 V for HER and OER respectively. Despite low non-precious metal loading, the Co3O4/Fe3O4 system showed high OER kinetic with low Tafel slope of 63 mV dec-1. Chronoamperometry tests performed, at 1.62 V and 0.30 V for OER and HER respectively, on the Co3O4/Fe3O4 catalyst demonstrated extremely stable OER over a period of 8 hours. FESEM images of Co3O4/Fe3O4 revealed that the Co3O4 platelets were self-assembled into edge-on orientation on the Fe3O4 support. This largely accounted for the high catalytic activities observed due to the large total exposed surface area of Co3O4/ Fe3O4 for catalytic reactions, besides electrochemical and morphological effects. Despite negating the need for specially tailored morphologies, Co3O4/Fe3O4 demonstrated enhanced electrochemical performance and stability. This may serve as a cost-effective route to the large-scale commercialization of electrolyzers and fuel cells via facile synthesis of non-precious metal oxides as the catalyst-support system for enhanced electrochemical water electrolysis. KEYWORDS: Hydrogen evolution, oxygen evolution, self-assemble, iron oxide, cobalt oxide, catalyst support, electrocatalyst, superparamagnetic.

INTRODUCTION Hydrogen and oxygen reactions commonly take place in renewable energy devices such as fuel cells, electrolyzers and metal-air batteries. Being kinetically sluggish, oxygen reactions (e.g. Oxygen Reduction Reaction (ORR) for fuel cells and Oxygen Evolution Reaction (OER) for water splitting electrolysers) still require high loading of precious metals (Platinum Group Metals, PGMs).1,2 This is the main hindrance to large-scale commercialization of renewable energy devices. Extensive research efforts have been devoted to replace PGMs with other less-precious metals. Co3O4 for example, is usually synthesized on catalyst supports with special morphologies in the forms of nanotubes, nanorods or ultra-thin 2D materials to improve its activities toward OER or HER.3-6 Cobalt oxide supported on graphene-based materials has been reported to be an active non-precious metal oxide catalyst for

OER.7-9 Nevertheless, non-precious metal catalysts usually require special morphologies to be compatible in performance with the PGMs for HER and OER. This being the trade-off for replacing the PGMs with low-cost nonprecious metal catalysts in fuel cells and electrolyzers. One of the major criteria for a catalyst support is that it must have high electrical conductivity to facilitate electron transfer. Nitrogen-doped (N-doped) graphene and N-doped carbon are widely used to improve the electrical conductivities of catalyst supports.10,11 In recent years, research also focuses on bi-functional catalysts which are capable of catalyzing more than one reactions, usually a combination of two from HER, OER and ORR.12,13 For this, the transition metal phosphides, oxides and sulphides have been actively investigated.14-16 For the kinetically sluggish OER, more than one types of metals (e.g: Ni-Fe-, Co-Fe-based catalysts) are used to improve its catalytic performance.17,18 The adsorption of OER intermediates (*OH, *O and *OOH) on the surface of a catalyst plays an

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important role in determining its kinetics.19 Several approaches have been taken to optimize the adsorption process. Among these, cobalt oxyhydroxide (CoOOH) was doped with Fe to optimize the adsorption energies of the intermediates on the catalyst surface.20 Despite the improved performance of these non-precious metal-based catalysts for hydrogen and oxygen reactions so far, having a stable bifunctional non-precious metal-based catalyst to enhance the efficiencies of both hydrogen and oxygen reactions in the same alkaline or acidic medium still remains a formidable challenge. Being cost effective and readily available, Fe is usually used to form oxides or oxyhydroxide compounds with other metals. However, iron oxide, especially superparamagnetic Fe3O4, has rarely been studied as a catalyst support. The stability of a catalyst is another important factor to consider in hydrogen and oxygen reactions, even for PGMs. Alloying phosphorus to non-precious metals is among the efforts taken to improve catalyst stabilities.21,22 For cobalt oxide, it has been reported that its surface layer gradually turned amorphous resulting in degradation of its performance.23 Therefore, there is a need for a stable catalyst support to enhance the stability of cobalt oxide catalyst. In this work, we used relatively inexpensive and readily available iron oxide as the catalyst support to reduce the metal loading of the more toxic and more active Co3O4 for OER and HER. Superparamagnetic Fe3O4 could potentially improve the catalytic activity of the kinetically sluggish OER by providing a more conductive support for the catalytically more active but electrically less conductive Co3O4. 24 This enhances the metal-metal interaction between the metal-based support and the supported Co3O4 catalyst. Fe3O4-supported Co3O4 catalyst can also be easily separated from the reaction medium for re-use. Besides high catalytic activity and stability, a simple and scalable catalyst synthesis method is crucial to large-scale commercialization of metal fuel cells and electrolyzers. There exist a number of established methods for iron oxide synthesis. Among these are; thermal decomposition, co-precipitation, hydrothermal and sol-gel methods.25-28 Co-precipitation is most commonly used to synthesize Fe3O4 nanoparticles using Fe2+ and Fe3+ ions (eg: FeSO4 or FeCl2 and FeCl3) as precursors.29 The precursors are mixed in a stoichiometric ratio for subsequent precipitation in a basic medium (eg: NaOH or KOH). Mixing of the precursors and the co-precipitation process are normally carried out under an inert atmosphere (in N2 or Ar) to prevent the oxidation of the Fe2+ precursor to Fe3+, which eventually leads to the formation of Fe2O3 instead of pure Fe3O4. Compared to other methods that use organic surfactants to control particle sizes, co-precipitation method usually has low-yield of Fe3O4 with relatively large particle sizes distributed over a wide range. The main advantage of co-precipitation method is simplicity and scalability in that it involves no organic solvent. The use of organic solvent also tends to block the active sites of iron oxide catalysts. Nucleation is the most important factor affecting Fe3O4 particle and crystallite sizes. This in turn will affect its magnetic properties.30 On the other hand, the

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pH of a precipitation medium has an important bearing on the co-precipitation process of Fe2+ and Fe3+ to form oxides.31 Non-uniform distribution in pH that arises when an alkaline solution is dropped into a precursor medium can potentially cause the formation of mixed FeOOH and Fe3O4.32 This decreases the yield of Fe3O4 and complicates subsequent Fe3O4 separation and extraction. It is important to consider these factors in the synthesis of superparamagnetic iron oxide as the catalyst for water electrolysis. In this study, superparamagnetic iron oxide (pure Fe3O4 phase) was synthesized using an improved coprecipitation process in an alkaline medium involving the nucleation of Fe3O4 under sonication to prevent agglomeration. The co-precipitation process was carried out, instead of in an inert environment, by preheating the iron precursor solution to 80oC before co-precipitation to reduce the dissolution of oxygen in the solution. Our present work aims to reduce cost by reducing catalyst loading, and replacing the relatively expensive Co3O4 with Fe3O4, whereas most of the published works aimed to achieve the highest possible catalytic activity using more sophisticated and expensive catalyst-support systems. EXPERIMENTAL SECTION Materials. Iron (II) Sulfate (FeSO4·7H2O, Friendemann Schmidt, 99%), Iron (ll) Chloride (FeCl3·6H2O, Friendemann Schmidt, 99%), Cobalt (ll) Nitrate (Co(NO3)2, Friendemann Schmidt, 99%), Sodium Hydroxide (NaOH, Emsure, 99%), Potassium Hydroxide (KOH pellets, Sigma-Aldrich, 99.99%), Nafion solution (SigmaAldrich, 5 wt%). Synthesis of Fe3O4 and α-Fe2O3 as catalysts and supports. For the synthesis of Fe3O4 as shown in Figure 1, the iron precursor solution was prepared by dissolving 0.6667 M of FeSO4·7H2O and 1 M of FeCl3·6H2O (mole ratio equals 2:3) in ultrapure water. The precursor solution was heated to 80oC before added drop-wise into 1 M NaOH under sonication. After one minute of precipitation, the black precipitate thus formed was immediately collected using a permanent magnet and rinsed in ethanol followed by ultrapure water under centrifugation. This rinsing process was repeated twice before the precipitate was left to dry overnight in an oven at 110oC. Similar experimental steps were involved in the preparation of Co3O4 and α-Fe2O3 up to the production stage of dried precipitate powders. For α-Fe2O3, 1 M FeCl3·6H2O solution was subsequently added drop-wise into 1 M NaOH to yield the precipitate of intermediate products. After rinsing and drying, the precipitate was pyrolysed in air inside a box furnace at 300oC for 4 hours to form the final product of α-Fe2O3. Synthesis of Co3O4 catalyst. As shown in Figure 1, Co3O4 catalyst was prepared by precipitating Co(OH)2 using stoichiometric proportions of 1 M Co(NO3)2 precursor solution and 1 M NaOH according to Equation 1. After rinsing and drying, the precipitate was calcinated in air


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Figure 1: Schematic flow diagrams for the synthesis of catalysts and catalyst-support systems.

inside a box furnace at 300oC for 3 hours to form Co3O4 (Equation 2).

Thermal decomposition of Co(OH)2:

Synthesis of Co3O4 catalyst on Fe3O4 or α-Fe2O3 support. The preparation of Fe3O4-supported Co3O4 (Co3O4/ Fe3O4) catalyst involved the addition of 0.3 g of Fe3O4 to 1.5 ml of 4.3 M Co(NO3)2 solution, followed by sonication for 45 minutes to impregnate the Fe3O4 with Co(NO3)2 solution to form Co(NO3)2 solution-impregnated Fe3O4 (Fig. 1). 13 ml of 1 M NaOH was then added slowly under sonication to the mixture to yield Co(OH)2 on Fe3O4. The as synthesized Co(OH)2 on Fe3O4 support was then rinsed in ethanol followed by ultrapure water. The rinsing process was repeated twice. The Co(OH)2 on Fe3O4 support was then left to dry overnight in a drying oven at 110oC and pyrolysed in a furnace at 300oC for 3 hours in air. This thermally decomposed Co(OH)2 into Co3O4 to form Co3O4/ Fe3O4. Similarly, α-Fe2O3-supported Co3O4 (Co3O4/α-Fe2O3) was synthesized using the same experimental steps as for Co3O4/ Fe3O4 described above, by replacing Fe3O4 with α-Fe2O3. In both cases, the final supported catalysts were constituted of 40 wt% Co3O4 on iron oxide (Fe3O4 or α-Fe2O3) supports.

For electrochemical characterisation, total catalyst loading on electrode was 250 µg/cm2 for all supported and unsupported catalysts. In the case of iron oxide supported Co3O4 catalysts, loading of the catalyst on electrode was 100 µg Co3O4/cm2 (40 wt% Co3O4). Catalyst ink was prepared by dispersing the catalyst in a mixture of Nafion, ultrapure (> 18.2 MΩ/cm) water and isopropanol (IPA). The volume ratio of water to IPA used was 1: 1 and that of Nafion to catalyst support was 0.2: 1 (based on 5 wt% Nafion in the Nafion solution). The catalyst ink was then sonicated for 1 hour. 5 µL of the catalyst ink was dropcasted onto a well-polished Glassy Carbon Electrode (GCE) of surface area 0.0314 cm2 and left to dry overnight.

Formation of Co(OH)2: CoNO 2 NaOH → CoOH 2 NaNO

[1]

6 CoOH O → 2 Co O 6 H O

[2]

All electrochemical characterizations, including those for OER and HER, on each catalyst were carried out in 0.1 M KOH at room temperature using a potentiostat (Autolab, PGSTAT302N Electrochemical Work Station, Methrohm AG, Switzerland). For HER and OER, the electrolyte was saturated with N2 (at a flow rate of 5 sccm) until a stable Open Circuit Potential (OCP) was achieved. OER curve was obtained by scanning the voltammogram from 0.26 V to 0.74 V vs. Ag/AgCl and HER from -0.96 to -1.76 V vs. Ag/AgCl, both at a scan rate of 10 mV s-1. At least 20



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sweeps were carried out for both the HER and OER scans. The final stable polarisation curve was taken to evaluate the OER and HER activities.

where k is a shape factor constant of value ~ 0.9 and β is peak broadening in radian given by FWHM after corrections for instrumental broadening and Kα doublet effect.33

Chronoamperometry test for each of the catalysts was carried out in 0.1 M KOH at room temperature in an electrochemical cell. Silicon (111) wafer was used as the substrate to deposit the catalyst ink prepared in the same way as for other tests on glassy carbon electrodes. The silicon substrate was oriented vertically in the electrolyte to remove the oxygen/hydrogen bubbles (by upward movement of the bubbles) formed on the catalyst surface, which otherwise would contribute to the shielding effect between the catalyst surface and the electrolyte. The electrochemical cell was saturated with N2 (at a flow rate of 5 sccm) until a steady OCP was achieved. The potential of the working electrode was stepped up from OCP to -1.27 V and 0.65 V vs Ag/AgCl to assess the HER and OER stabilities respectively. Each potential was held for 8 hours to monitor the changes in current density over the measuring time.

Using lanthanum hexaboride (LaB6) (SRM 660) as the standard, a value of 0.06o was estimated from the FWHM of the diffractogram obtained in the same measurement range. This was used as the correction for instrumental broadening. Kα doublet effect was corrected using the on-line computer program to eliminate the Kα2 radiation based on the algorithm described by Landell et al (1975).34

The Electrochemical Impedance Spectroscopy (EIS) or AC Impedance measurements were carried out potentiostatically in 0.1 M KOH under N2, at an AC amplitude of 0.2 V from 0.1 Hz to 100 kHz and at 10 points per decade. Capacitance measurements were performed by Cyclic Voltammetry (CV) in o.1 M KOH under N2 at different scan rates (10, 20, 40, 60, 80 and 100 mVs-1) in the potential window of 0-0.6 V vs. Ag/ AgCl. Conversion of reference electrode potential vs. Ag/ AgCl to that vs. RHE (Reversible Hydrogen Electrode) was based on the Nernst equation by which 0.1 M KOH is equivalent to pH 13. / was taken to be +0.197 V. All the potentials reported were iR-corrected. E = E/ 0.059 pH E/ &

[3]

For X-ray diffractometry (XRD) phase analysis, each catalyst sample was packed into the cavity of a zerobackground Si sample holder using the front-loading technique. X-ray measurements were performed on a Panalytical X-Pert Pro MPD diffractometer operating at 40 kV and 30 mA using the CuKα radiation. X-ray diffraction was also used to monitor the structural changes during heating at various temperatures ranging from 150 to 400oC and a heating rate of 10oC min-1. The phases of the materials present in the samples were identified by matching the observed diffractograms with reference patterns from PDF-2 Powder Diffraction File database, using the commercial search/match program from Panalytical. Average crystallite size or lattice coherence length，D， was determined from the full-width-at-half maximum (FWHM) of a single Bragg peak profile using the Scherrer Equation given by:

β = kλ/( D cos θ )

[4]

Microstructural analysis was carried out using a HRTEM (JEM 2100-F with accelerating voltage of 200 kV) and FESEM (Hitachi SU 8230). RESULTS AND DISCUSSION Microstructural Characterizations. Figure 2a shows the typical XRD diffraction patterns of the catalysts and catalyst-supports. All diffraction peaks from the assynthesized Co3O4 catalyst are indexed to the space group 227 (Fd-3m) (PDF No: 01-074-1657) of the cubic structure. The as-synthesized Fe3O4 shows diffraction peaks that match the cubic structure of space group 227 (Fd-3m) (PDF No: 01-071-6336). For α-Fe2O3, the diffraction peaks correspond well with the rhombohedral structure of space group 167 (R-3C) (PDF No: 00-001-1053). From Rietveld refinement analysis, the percentage weight of Co3O4 in both Co3O4/ Fe3O4 and Co3O4/α-Fe2O3 was about 40 wt% (Fig. S2a and S2b). On a gross scale, the sharp diffraction peaks observed would imply good crystalline nature of all the samples under investigations. Crystallite sizes of the samples were calculated using the Scherrer equation (Eq. 4) based on the highest diffraction peak obtained. The crystallite sizes of the unsupported Co3O4, Fe3O4 and αFe2O3 catalysts obtained were approximately 24 nm, 12 nm and 15 nm respectively. Besides maintaining the same crystal structure, as may be evidenced from the X-ray diffractograms in Figure 2a, Fe3O4 and α-Fe2O3 supports almost maintained their original crystallite sizes of 12 nm and 14 nm respectively after forming Co3O4/Fe3O4 and Co3O4/α-Fe2O3 catalyst-support systems. This indicates that precipitation and calcination of Co3O4 on presynthesized iron oxide (Fe3O4 and α-Fe2O3) supports had negligible effect on the structures and crystallite sizes of the supports. The Fe3O4 support was stable up to 350oC, as seen from the non-ambient X-ray diffractograms in Figure S1a. This shows that Fe3O4 phase of the support was maintained during calcination of Co(OH)2 on Fe3O4 support at 300oC to form Co3O4/ Fe3O4. The assynthesized Fe3O4 also had a reasonably small average particle size of ~15 nm (Fig. S4b) capable of exhibiting superparamagnetic property as seen in Figure S3. Fe3O4 was obtained in pure phase with 100%-yield using a modified co-precipitation procedure under sonication. High yield of Fe3O4 was accomplished by adding the Fe3O4 precursor solution to 1 M NaOH, instead of the latter to the former (as normally practiced) to ensure a uniform pH throughout the precursor solution. From the results of non-ambient XRD analysis in Figures S1a and S1b, the


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Figure 2: (a) XRD patterns of the as-synthesised catalysts and catalyst-support systems. (b) Bright-field HRTEM image of Co3O4/ Fe3O4. (Inset: Particle size distribution of the Fe3O4 support), (c) Magnified image of (b) (Inset: SADP from the imaged area) and (d) HRTEM image showing lattice fringes of Co3O4 (022) and Fe3O4 (113) planes. FESEM images showing the morphologies of (e) Co3O4 (Inset: Magnified image from a selected area showing porous structure of Co3O4), (f) Fe3O4 and (g) Co3O4/ Fe3O4 with the Co3O4 viewed edge-on (Inset: Enlarged image of Co3O4 supported edge-on the Fe3O4).

heating temperatures for Fe3O4 and Co(OH)2 seemed to have minimum effect on the crystallite sizes of their final products. This implies that the major factors that determined the sizes of the catalysts might have originated from the precipitation stage. These factors include the type of precursor used, reaction time and pH of the precipitation medium. Rod-shaped α-Fe2O3 measuring 650 nm in length (Fig. S4a) were obtained using the same co-precipitation method and conditions as in the case of Fe3O4. Figure 2b, 2c and 2d are the HRTEM images and Figure 2g the FESEM images of a typical Co3O4/ Fe3O4 catalyst-support system. Figures 2e and 2f are the FESEM images of the unsupported Co3O4 catalyst and Fe3O4 support respectively. The particle size of the superparamagnetic Fe3O4 support as determined from the HRTEM image in Figure 2b was approximately 15 nm. The selected area diffraction pattern (SADP) in Figure 2c is typical of polycrystalline Co3O4/ Fe3O4. Figure 2d shows the (022) and (113) lattice fringes from a single crystal of Co3O4 and Fe3O4. The synthesis of Co3O4 with controlled morphologies usually involves organic solvents such as oleic acid and oleylamine.35 In this work, we used Co(NO3)2 as the starting material and NaOH as the precipitation medium, followed by subsequent calcination at 300oC in air to obtain Co3O4 from Co(OH)2 precursor. The as-synthesized Co3O4 exhibits uniform and porous hexagonal-platelets which are mostly oriented with the hexagonal planes facing upwards as evidenced from the FESEM image in Fig. 2e. The sizes of the Co3O4 platelets are ~150 – 200 nm, measured

diagonally across the platelets. Porous structure of Co3O4 might be caused by the removal of volatile components during thermal decomposition.36 When supported on Fe3O4, most of the hexagonal platelets were orientated edge-on on the Fe3O4 particles as seen in the FESEM image in Figure 2g. The Fe3O4 seemed to provide a good physical support to the hexagonal Co3O4 platelets. These monodispersed and self-assembled Co3O4 platelets should provide a larger total exposed surface area for enhanced catalytic activities compared to the unsupported Co3O4. Compare with iron oxide supported Co3O4, unsupported iron oxide particles and Co3O4 platelets tend to agglomerate (Fig. 2e and 2f). Consequently, we can reasonably deduce that incorporating two morphologically different catalytic materials and self-assembly of the catalyst on its support help in reducing agglomeration leading to an increased total exposed surface area for enhanced catalytic performance. Electrochemical Characterizations. We report, for the first time, the water electrolysis performance of Co3O4 on iron oxide catalyst-support system. Electrochemical tests were carried out on all the unsupported and supported catalysts. For a more consistent and systematic comparison, the overall catalyst loading for each catalyst on the electrode was maintained at 250 µg/cm2. Current density was calculated according to the geometric area of the glassy carbon electrode used. A blank Glassy Carbon Electrode (GCE) without catalyst was used to detect the background electrochemical activities. It showed insignificant OER and HER catalytic activities. From Figure 3a and 3d,



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-0.35 117 mV dec-1

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Figure 3: Steady-state GCE measurement of (a) OER and (d) HER in N2-saturated 0.1 M KOH electrolyte with a scan rate of 10 2 2 mV/s and total catalyst loading of 250 µg/cm (or 100 µgCo3O4/cm for Co3O4/ Fe3O4 and Co3O4/ α-Fe2O3) (Insets: low current density regions). The OER Tafel slopes of the catalysts for (b) OER and (e) HER. 8-hour chronoamperometry at (c) 1.62 V and (f) 2 -0.30V in N2-saturated 0.1 M KOH with total catalyst loading on silicon (111) substrate maintained at 250 µg/cm .

it can be seen that the addition of Fe3O4 catalyst support increases the kinetics of both HER and OER compared to pure or unsupported Co3O4. This can be inferred from the decrease in the overpotentials of 0.22 V and 0.06 V for HER and OER respectively. The noise generated at the “tail” of HER graphs was due to vigorous generation of H2 bubbles that covered the catalyst’s surface at high HER overpotentials. The onset potential of Fe3O4-supported Co3O4 catalyst for OER was 1.59 V. This required a potential of 1.64 V to achieve a current density of 10 mA/cm2, which was the standard figure-of-merit used to access OER or HER (Fig. 3a). In the absence of Fe3O4 support, the Co3O4 catalyst showed significantly lower OER activity compared to urchin-like Co3O4 sphere reported elsewhere.37 However, its catalytic activity was greatly improved with Fe3O4 support. Although the catalytic activity of the Fe3O4 catalyst was lower than that of the α-Fe2O3 (Fig. 3a), as a catalyst support for Co3O4 (in Co3O4/ Fe3O4 system), it showed better OER performance compared to the α-Fe2O3 -supported Co3O4 (Co3O4/ α-Fe2O3) system. This can be understood from the following arguments; Since Co3O4 was an active catalyst material for OER, as evidenced from the much higher kinetics associated with Co3O4 catalyst in Figure 3a, the electrical conductivity of the catalyst support played a more important role in de-

termining the kinetic of OER than the catalytic activity contributed by the catalyst support. Also, as the Fe3O4 support offered higher electrical conductivity compared to α-Fe2O3',24 it was a more efficient catalyst support than α-Fe2O3. Although the OER catalytic activity of Co3O4/ Fe3O4 in this work is comparable to that of the Co3O4 catalyst on carbon-based support with special morphology (Co3O4/N-rmGO),7 the catalyst loading we used was about four times lower (250 µg/ cm2 vs. 1000 µg/cm2 of overall catalyst loading). Besides this, it was also more superior to the nitrogen-doped graphene-cobalt oxide nanoparticle (Co-N/G) system for OER with comparable catalyst loading.38. Bearing in mind that, in general, the onset and kinetic of a catalytic activity may be shifted favourably with higher catalyst loading, up to the limitation of mass transport.39 The OER polarization curves within the dashed lines in Figure 3a for the cobalt-containing catalysts, might be the Co2+/ Co3+ peak from Co(OH)2 to CoOOH conversion.20 Another small peak after that of Co2+/ Co3+ from the unsupported Co3O4 may be assigned to Co3+/Co4+,40 which has been masked off in the presence of iron oxide support. Cobalt containing catalysts showed higher OER performances. This was achieved via the enhancement in the interaction of CoOOH with the OHthat formed during OER.41 Iron oxide support increased


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the electrical conductivity of the catalyst-support system as can be evidenced from the EIS results of AC impedance measurements in the next section. For the popularly studied iron-doped metal oxyhydroxides (e.g: Ni(Fe)OxHy or Co(Fe)OxHy), it is still disputable as whether Fe acts as the active site for OER in iron-doped oxyhydroxides since Fe could either increase the number of active sites of its host, metal oxyhydroxides,42 or it can be the active site itself and be more active than its host metal oxyhydroxides in the iron-doped metal oxyhydroxide system.43 However, for the iron oxide-supported Co3O4 system in our work, we believe that majority of the active sites for OER came from the active Co species because the enhancement of the Co2+/ Co3+ peak in the iron oxide-supported Co3O4 catalysts led to an increase in OER. Tafel slopes were used to analyse the kinetics of the catalyst activities and the catalyst-support systems. As seen in Figure 3b, Co3O4/ Fe3O4 catalyst- support system exhibits the highest OER kinetic represented by lowest Tafel slope. Its OER Tafel slope of 63 mV/ dec, is slightly lower than that of the Co3O4/ N-rmGO and Co3O4/ rmGO.7 Besides this, its OER performance is better (lower onset and Tafel slope) than that of Ni-MOF and is comparable to that of Ni at graphene.44 The use of Fe3O4 as the catalyst support for Co3O4 decreases the overpotential of OER by 0.06V and Tafel slope by 3 mV dec-1 compared to the unsupported Co3O4 catalyst. From Figure 3d, it can be seen that being the best catalyst for OER among all the catalysts tested in this work, Co3O4/ Fe3O4 is also the best performed catalyst for HER. It has an onset potential of -0.23 V and a potential of -0.36 V to achieve a current density of 10 mA/cm2. The peak at 0.037 V (for anodic scan of HER) is clearly visible for the α-Fe2O3 and Fe3O4 catalysts. This is the Fe2+/ Fe3+ peak,45 shown in the low overpotential region before the onset of HER in Figure 3d. The HER performance for α-Fe2O3 and Fe3O4 catalysts are significantly close to each other since majority of the active sites for HER are from iron.46 This can be inferred from the higher HER activities of the iron oxides observed, compared to the cobalt oxides. Moreover, the fact that a decrease in the current density of the Fe2+/Fe3+ peak leads to a decrease in HER (Fig. S6e and S6f) would further enhance our claim that the Fe species were the active sites for HER for α-Fe2O3 and Fe3O4. The Fe2+/ Fe3+ peak is absent from the iron oxide-supported Co3O4 catalysts (Co3O4/ α-Fe2O3 and Co3O4/ Fe3O4). This was caused by the inhibition of iron dissolution and oxidation by Co3O4 which was an indication of good electronic interactions between the Fe-based supports and the Co-based catalysts. Interestingly, the inhibition of iron oxidation increased the HER activity, meaning that Fe2+/Fe3+ oxidation was not the active site for Co3O4/ αFe2O3 and Co3O4/ Fe3O4, although the real active sites for these catalyst-support systems still remain unknown. The iron oxide-supported Co3O4 catalysts (Co3O4/ Fe3O4 and Co3O4/ α-Fe2O3) had similar performances towards HER. Co3O4/Fe3O4 shows a lower overpotential compared to Co3O4/ α-Fe2O3. Nevertheless, it has slightly higher Tafel slope of 117 mV dec-1 compared to 102 mV dec-1 for Co3O4/

α-Fe2O3 (Fig. 3e). Co3O4 itself was a less active catalyst for HER compared to other iron-containing catalysts although it was active towards OER. Comparing the HER activities after OER, the HER activity of the unsupported Co3O4 increases after 20 sweeps of OER (Fig. S6d), while that of the Co3O4/ Fe3O4 and Co3O4/ α-Fe2O3 remains relatively unchanged (Figs. S6a & S6c). Conversely, the HER activities of iron oxides (Fe3O4 and αFe2O3) decrease after 20 sweeps of OER (Figs. S6e & S6f). For the comparison of OER after HER, the enhancement in the OER activity of Co3O4/ Fe3O4 after 20 sweeps of HER (Fig. S6b) agrees well with the claim that an increase in the ratio of Co2+/ Co3+ after HER could enhance the activity of OER.10 Therefore, we can deduce that the Co3O4-iron oxides catalyst-support systems helped in stabilising the HER activities after OER and enhancing the OER activities after HER. If we focus on the Fe2+/ Fe3+ oxide/ oxyhydroxide peak before the onset of HER, the peak height is relatively lower after the OER (Fig. S6e & S6f). This is indicative of a decrease in the amount of the iron redox species which subsequently led to a decrease in the HER activity of the Fe3O4 and α-Fe2O3 catalysts after OER. However, the potential at which the Fe2+/ Fe3+ oxide/ oxyhydroxide peak occurs remains unchanged. This implies that the active sites for both HER and OER were the same for Fe3O4 and α-Fe2O3. For the case of Co3O4 (Fig. S6d), significant enhancement of HER after OER might be caused by the formation of Co (oxy)hydroxides during OER that also served as the active sites for HER. The stability of OER and HER are often tested by repetitively cycling the voltammogram at the potential windows of HER and OER. For the case of OER, certain binding energy of metal cation with OH- is optimum to OER and the binding energy can change with the oxidation number of metal.47 Potential cycling can help in recovering the oxidation number back to optimum during the reverse cycle. Therefore, this testing cannot provide a complete picture on the stability of the catalyst since the oxidised species can be recovered during the backward cycle or reductive polarization. For both the OER and HER stability measurements in this work, the catalyst ink for Co3O4/ Fe3O4 was deposited onto silicon (111) substrate which was mounted at 90o w.r.t. the electrolyte surface to prevent the trapping of air bubbles on the surface of the electrode (as has been explained in the section on methodology). Measurements were carried out on OER and HER stabilities to obtain the current density changes from chronoamperometry curves throughout the 8-hour chronoamperometry at 1.62 V (oxygen evolution potential in Fig. 3c) and -0.30 V (hydrogen evolution potential in Fig. 3f). Surprisingly, Co3O4/ Fe3O4 was very stable towards OER for 8 hours of chronoamperometry at 1.62 V with negligible drop in current density. For chronoamperometry at -0.30 V, there was an initial 33-minute plateau (from 3036 s to 5000 s) of stable HER, followed by an increase in current density. The increase in current density might be caused by an increase in the active sites after the initial reduction of surface oxides or oxyhydroxides during hydrogen evolution, followed by gradual stabilisation of the active sites.


ACS Applied Nano Materials FESEM images after HER and OER polarisations show insignificant morphological changes for the Co3O4/ Fe3O4 catalysts after 8 hours of chronoamperometry test at both hydrogen and oxygen evolution potentials (Figs. S7a and S7b). The hexagonal shape Co3O4 platelets were retained after chronoamperometry. The capacitance and AC impedance measurements on Co3O4/ Fe3O4 and the unsupported catalysts (Co3O4 and Fe3O4) could provide some insights into the role of Fe3O4 support in affecting the surface area available for ion adsorption/ desorption and electrical conductivity of Co3O4/ Fe3O4. The capacitance measurements were performed by obtaining CV at different scan rates (Fig.S8), in the potential window before OER. The capacitance were calculated according to equation S8. From Fig. 4a, we noticed that the capacitance of Co3O4/ Fe3O4 is about two orders of magnitude larger than that of the Co3O4 and an order of magnitude higher than that of Fe3O4. This shows that the edge-0n orientation of Co3O4 on its Fe3O4 support plays important role in increasing the surface area of Co3O4/ Fe3O4 for ion adsorption/ desorption as compared to the unsupported Co3O4 catalysts. The Nyquist plot at AC amplitude of 0.2 V (Fig. 4b) shows low charge transfer resistance for Co3O4/ Fe3O4 catalyst (this could be more clearly observed at the high frequency region shown in the inset) that indicates good electrical conductivity. The high surface area and electrical conductivity of Co3O4/ Fe3O4 are among the factors that lead to the high catalytic activity observed.

Capacitance (mAh g-1)

(a) 10-1 10-3 10-5 10-7 10-9

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The Nyquist plot at the AC amplitude of 0.2 V (Inset: High frequency region). (Y-axes are in log scale) CONCLUSION Co3O4 on iron oxides catalyst-supports were synthesized using a simple precipitation process under sonication without the use of organic solvent. Pure phase of Fe3O4 was obtained via a modified co-precipitation procedure without the need of an inert environment. Compared to unsupported Co3O4, the Co3O4 on iron oxides catalystsupports showed better performance as a bifunctional catalyst for electrochemical alkaline water electrolysis. A typical Co3O4/ Fe3O4 catalyst-support system demonstrated good HER and OER over a period of 8 hours. Iron oxide catalyst supports, especially Fe3O4, played an important role in electrical conductivity enhancement of the systems, besides being the co-catalyst to Co3O4 by virtue of the excellent chemical and electronic interactions between iron and cobalt during OER and HER. Selfassembly of edge-on oriented Co3O4 platelets on Fe3O4 support and the morphological differences (shape factor) between them also helped in reducing the common problem of particle agglomeration. This provided a large total exposed surface area of the Co3O4/Fe3O4 system for enhanced catalytic reactions. These synergistic effects between the Co3O4 catalyst and its iron oxide support are highly appropriate for bifunctional catalytic reactions (HER and OER) in water electrolysis. In general, good material selection on a catalyst support helps in exploiting the natural material properties to enhance catalytic activities without having to tune the material properties via special morphological modifications of the material. This normally requires sophisticated synthesis method. In the lights of the above arguments, further improvement in the catalytic performance for water electrolysis may be achieved via our simple materials synthesis route, negating the need for material morphological tuning.

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ACKNOWLEDGMENT This work was supported by University of Malaya RU Grant (RU018E-2016)

AUTHOR INFORMATION Corresponding Author

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* E-mail: [email protected]

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Supporting Information

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Figure 4: (a) Specific capacitance of the catalysts from cyclic voltammetry at different scan rates (Figure S8). (b)

The supporting information is available free of charge on the ACS Publications website at http://pubs.acs.org.


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ACS Applied Nano Materials Non- ambient XRD analysis, Rietveld refinement analysis, M-H curves from Vibrating Sample Magnetometer (VSM), HRTEM and FESEM images, polarization curves of HER after OER (vice versa) and CV at different scan rates.

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REFERENCES (1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

Stamenkovic, V. R.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M. Energy and Fuels from Electrochemical Interfaces. Nat. Mater. 2017, 16, 57-69. Reier, T.; Oezaslan, M.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal. 2012, 2, 1765-1772. Chang, J.; Xiao, Y.; Xiao, M.; Ge, J.; Liu, C.; Xing, W. Surface Oxidized Cobalt- Phosphide Nanorods as an Advanced Oxygen Evolution Catalyst in Alkaline Solution. ACS Catal. 2015, 5, 6874-6878. Li, X.; Zhang, L.; Huang, M.; Wang, S.; Li, X.; Zhu, H. Cobalt and Nickel Selenide Nanowalls Anchored on Graphene as Bifunctional Electrocatalysts for Overall Water Splitting. J. Mater. Chem. A. 2016, 4, 14789-14795. Ai, L.; Tian, T.; Jiang, J. Ultrathin Graphene Layers Encapsulating Nickel Nanoparticles Derived Metal–Organic frameworks for Highly Efficient Electrocatalytic Hydrogen and Oxygen Evolution Reactions. ACS Sustainable Chem. Eng. 2017, 5, 4771-4777. Hou, C.; Cao, S.; Fu, W.; Chen, Y. Ultrafine CoP Nanoparticles Supported on Carbon Nanotubes as Highly Active Electrocatalyst for Both Oxygen and Hydrogen Evolution in Basic Media. ACS Appl. Mater. Interfaces. 2015, 7, 2841228419. Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co₃O₄ Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780-786. Shahid, M. M.; Rameshkumar, P.; Basirun, W. J.; Juan, J. C.; Huang, N. M. Cobalt Oxide Nanocubes Interleaved Reduced Graphene Oxide as an Efficient Electrocatalyst for Oxygen Reduction Reaction in Alkaline Medium. Electrochimica Acta. 2017, 237, 61-68. Liu, X. X.; Zang, J. B.; Chen, L.; Chen, L. B.; Chen, X.; Wu, P.; Zhou, S. Y.; Wang, Y. H. A Microwave-Assisted Synthesis of CoO@Co Coreshell Structures Coupled with Ndoped Reduced Graphene Oxide Used as a Superior Multifunctional Electrocatalyst for Hydrogen Evolution, Oxygen Reduction and Oxygen Evolution Rreactions. J. Mater. Chem. A. 2017, 5(12), 5865-5872. Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. In Situ Cobalt- Cobalt Oxide/ N- Doped Carbon Hybrids as Superior Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution. J. Am. Chem. Soc. 2015, 137(7), 2688-2694. Zhang, T.; He, C.; Sun, F.; Ding, Y.; Wang, M.; Peng, L.; Wang, J.; Lin, Y. Co3O4 Nanoparticles Anchored on Nitrogen- Doped Reduced Graphene Oxide as a Multifunctional Catalyst for H2O2 Reduction, Oxygen Reduction and Evolution Reaction. Sci. Rep. 2017, 7, 1-11. Chang, J.; Liang, L.; Li, C.; Wang, M.; Ge, J.; Liu, C.; Xing, W. Ultrathin Cobalt Phosphide Nanosheets as Efficient Bifunctional Catalysts for a Water Electrolysis Cell and the Origin for Cell Performance Degradation. Green Chem. 2016, 18, 2287-2295. Liu, S.; Li, L.; Ahn, H. S.; Manthiram, A. Delineating the Roles of Co3O4 and N-Doped Carbon Nanoweb (CNW) in Bifunctional Co3O4 /CNW Catalysts for Oxygen Reduction

(16)

(17)

(18)

(19)

(20)

(21)

(22)

(23)

(24)

(25)

(26)

(27)

(28)

and Oxygen Evolution Reactions. J. Mater. Chem. A. 2015, 3, 11615-11623. Liang, X.; Zhang, D.; Wu, Z.; Wang, D. The Fe- promoted MoP Catalyst with High Activity for Water Splitting. Appl. Catal., A. 2016, 524, 134-138. Prasad, P. P.; Moni, K. D.; Velikokhatnyi, O. I.; Kuruba, R.; Damodaran, K.; Jampani, P.; Gattu, B.; Pavithra, M. S.; Damle, S. S.; Kumta, P. N. Noble Metal- Free Bifunctional Oxygen Evolution and Oxygen Reduction Acidic Media Electro-Catalysts. Sci. Rep. 2016, 6, 1-14. Zhu, W.; Yue, X.; Zhang, W.; Yu, S.; Zhang, Y.; Wang, J.; Wang, J. Nickel Sulfide Microsphere Film on Ni Foam as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Commun. 2016, 52, 1486-1489. Bau, J. A.; Luber, E. J.; Buriak, J. M. Oxygen Evolution Catalyzed by Nickel–Iron Oxide Nanocrystals with a Nonequilibrium Phase. ACS Appl. Mater. Interfaces. 2015, 7, 19755-19763. Indra, A.; Menezes, P. W.; Sahraie, N. R.; Bergmann, A.; Das, C.; Tallarida, M.; Schmeißer, D.; Strasser, P.; Driess, M. Unification of Catalytic Water Oxidation and Oxygen Reduction Reactions: Amorphous Beat Crystalline Cobalt Iron Oxides. J. Am. Chem. Soc. 2014, 136, 17530-17536. Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Nørskov, J. K. Electrolysis of Water on Oxide Surfaces. J. Electroanal. Chem. 2007, 607, 83-89. Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. Cobalt- Iron ( Oxy) hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 3638-3648. Vigil, J. A.; Lambert, T. N.; Christensen, B. T. Cobalt Phosphide-Based Nanoparticles as Bifunctional Electrocatalysts for Alkaline Water Splitting. J. Mater. Chem. A. 2016, 4, 7549-7554. Pan, Y.; Liu, Y.; Liu, C. Nanostructured Nickel Phosphide Supported on Carbon Nanospheres: Synthesis and Application as an Efficient Electrocatalyst for Hydrogen Evolution. J. Power Sources. 2015, 285, 169-177. Kim, N. -I.; Sa, Y. J.; Cho, S. -H.; So, I.; Kwon, K.; Joo, S. H.; Park, J. -Y. Enhancing Activity and Stability of Cobalt Oxide Electrocatalysts for the Oxygen Evolution Reaction via Transition Metal Doping. J. Electrochem. Soc. 2016, 163, F3020-F3028. Zhang, Z.; Boxall, C.; Kelsall, G. H. Photoelectrophoresis of Colloidal Iron Oxides 1. Hematite (α-Fe2O3). Colloids Surf. Physicochem. Eng. Aspects. 1993, 73, 145-163. Alp, E.; Aydogan, N. A Comparative Study: Synthesis of Superparamagnetic Iron Oxide Nanoparticles in Air and N2 Atmosphere. Colloids Surf. Physicochem. Eng. Aspects. 2016, 510, 205-212. Jia, C.; Sun, L. D.; Luo, F.; Han, X.; Heyderman, L. J.; Yan, Z.; Yan, C.; Zheng, K.; Zhang, Z.; Takano, M.; Hayashi, N.; Eltschka, M.; Kläui, M.; Rüdiger, U.; Kasama, T.; CerveraGontard, L.; Dunin-Borkowski, R.; Tzvetkov, G.; Raabe, J. Large- Scale Synthesis of Single- Crystalline Iron Oxide Magnetic Nanorings. J. Am. Chem. Soc. 2008, 130, 1696816977. Cui, H.; Liu, Y.; Ren, W. Structure Switch Between αFe2O3, γ- Fe2O3 and Fe3O4 During the Large Scale and Low Temperature Sol–Gel Synthesis of Nearly Monodispersed Iron Oxide Nanoparticles. Adv. Powder Technol. 2013, 24, 93-97. Yu, W. W.; Falkner, J. C.; Yavuz, C. T.; Colvin, V. L. Synthesis of Monodisperse Iron Oxide Nanocrystals by Thermal Decomposition of Iron Carboxylate Salts. Chem. Commun. (Cambridge, England). 2004, 20, 2306-2307.



(29) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Vander Elst, L.; Muller, R. N. Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization, Physicochemical Characterizations, and Biological Applications. Chem. Rev. 2010, 110, 2574-2574. (30) Iwasaki, T.; Mizutani, N.; Watano, S.; Yanagida, T.; Kawai, T. Size Control of Magnetite Nanoparticles by Organic Solvent- Free Chemical Coprecipitation at Room Temperature. J. Exp. Nanosci. 2010, 5, 251-262. (31) Maria, C. M.; Pei, Y.; Ring, T. A. Room Temperature CoPrecipitation Synthesis of Magnetite Nanoparticles in a Large pH Window with Different Bases. Materials, 2013, 6, 5549-5567. (32) Ahn, T.; Kim, J. H.; Yang, H.; Lee, J. W.; Kim, J. Formation Pathways of Magnetite Nanoparticles by Coprecipitation Method. J. Phys. Chem. C. 2012, 116, 6069-6076. (33) Scherrer, P. Estimation of the Size and Internal Structure of Colloidal Particles by Means of Röntgen Rays. Nachr. Ges. Wiss. Göttingen Maths-Phys. KI. 1918, 2, 96 – 100. (34) Landell, J.; Zagofsky, A.; Pearlman, S. CuKα2 Elimination Algorithm. J. Appl. Cryst. 1975, 8, 499 – 506. (35) Zhang, D.; Zhu, J.; Zhang, N.; Liu, T.; Chen, L.; Liu, X.; Ma, R.; Zhang, H.; Qiu, G. Controllable Fabrication and Magnetic Properties of Double-Shell Cobalt Oxides Hollow Particles. Sci. Rep. 2015, 5, 1-6. (36) Su, D.; Dou, S.; Wang, G. Single Crystalline Co3O4 Nanocrystals Exposed with Different Crystal Planes for Li-O2 Batteries. Sci. Rep. 2014, 4, 1-9. (37) Xu, Q.; Su, Y.; Wu, H.; Cheng, H.; Hui, Y.; Li, N.; Liu, Z. Effect of Morphology of Co3O4 for Oxygen Evolution Reaction in Alkaline Water Electrolysis. Curr. Nanosci. 2015, 11, 107-112. (38) Wang, Q.; Hu, W.; Huang, Y. Nitrogen Doped Graphene Anchored Cobalt Oxides Efficiently Bi- Functionally Catalyze Both Oxygen Reduction Reaction and Oxygen Revolution Reaction. Int J Hydrogen Energy. 2017, 42, 5899-5907. (39) Li, X.; Zhang, H. J.; Li, H.; Deng, C.; Yang, J. Evaluation of Loading Influence on Catalytic Performance of Co- Based Catalyst for Oxygen Reduction. ECS Electrochem. Lett. 2014, 3, H33-H37. (40) Qi, J.; Zhang, W.; Cao, R. Aligned Cobalt- Based Co@CoO x Nanostructures for Efficient Electrocatalytic Water Oxidation. Chem. Commun. 2017, 53, 9277-9280. (41) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in Activity for the Water Electrolyser Reactions on 3d M (Ni, Co, Fe, Mn) Hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550-557. (42) Candelaria, S. L.; Bedford, N. M.; Woehl, T. J.; Rentz, N. S.; Showalter, A. R.; Pylypenko, S.; Bunker, B. A.; Lee, S.; Reinhart, B.; Ren, Y.; Ertem, S. P.; Coughlin, E. B.; Sather, N. A.; Horan, J. L.; Herring, A. M.; Greenlee, L. F. Multicomponent Fe– Ni Hydroxide Nanocatalyst for Oxygen Evolution and Methanol Oxidation Reactions Under Alkaline Conditions. ACS Catal. 2017, 7, 365-379. (43) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.; Sokaras, D.; Weng, T.; AlonsoMori, R.; Davis, R. C.; Bargar, J. R.; Nørskov, J. K.; Nilsson, A.; Bell, A. T. Identification of Highly Active Fe Sites in (Ni, Fe)OOH for Electrocatalytic Water Splitting. J. Am. Chem. Soc. 2015, 137, 1305-1313. (44) Ai, L.; Tian, T.; Jiang, J. Ultrathin Graphene Layers Encapsulating Nickel Nanoparticles Derived Metal–Organic Frameworks for Highly Efficient Electrocatalytic Hydrogen and Oxygen Evolution Reactions. ACS Sustainable Chem. Eng. 2017, 5, 4771-4777.

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(45) Lyon, J. L.; Stevenson, K. J. Anomalous Electrochemical Dissolution and Passivation of Iron Growth Catalysts in Carbon Nanotubes. Langmuir : The ACS Journal of Surfaces and Colloids. 2007, 23, 11311-11318. (46) Hedenstedt, K.; Simic, N.; Wildlock, M.; Ahlberg, E. Kinetic Study of the Hydrogen Evolution Reaction in Slightly Alkaline Electrolyte on Mild Steel, Goethite and Lepidocrocite. J. Electroanal. Chem. 2016, 783, 1-7. (47) Chen, G.; Zhou, W.; Guan, D.; Sunarso, J.; Zhu, Y.; Hu, X.; Zhang, W.; Shao, Z. Two Orders of Magnitude Enhancement in Oxygen Evolution Reactivity on Amorphous Ba0.5Sr0.5Co0.8Fe0.2O3− δ Nanofilms with Tunable Oxidation State. Sci. Adv. 2017, 3, 1-8.


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