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Porous NiFe-oxide Nanocubes as Bifunctional Electrocatalyst for Efficient Water Splitting Ashwani Kumar, and Sayan Bhattacharyya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14096 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 11, 2017

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ACS Applied Materials & Interfaces

Porous NiFe-oxide Nanocubes as Bifunctional Electrocatalyst for Efficient Water Splitting Ashwani Kumar and Sayan Bhattacharyya* Department of Chemical Sciences, and Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur - 741246, India *Email for correspondence: [email protected] Abstract Electrocatalytic water splitting, a combination of oxygen and hydrogen evolution reactions (OER and HER), is highly attractive in clean energy technologies especially for highpurity hydrogen production, whereas developing stable, earth-abundant, bifunctional catalysts has continued to pose major challenges. Herein a mesoporous NiFe-oxide nanocube (NiFe-NC) system is developed from NiFe prussian blue analog metal-organic framework, as an efficient bifunctional catalyst for overall water splitting. The NiFe-NCs with ~200 nm side length have Ni:Fe molar ratio of 3:2 and is a composite of NiO and α/γ-Fe2O3. The NCs demonstrate overpotentials of 271 and 197 mV for OER and HER, respectively in 1M KOH at 10 mA/cm2 which outperform those of 339 and 347 mV for the spherical NiFe-oxide nanoparticles (NPs) having similar composition. The electrolyzer constructed using NiFe-oxide nanocubes requires an impressive cell voltage of 1.67 V to deliver a current density of 10 mA/cm2. Along with a mesoporous structure with broad pore size distribution, the NiFe-NCs demonstrate the qualities of a desired corrosion-resistant water splitting catalyst with long term stability. The exposure of active sites at the edges and vertices of the NCs was validated to play a crucial role in their overall catalytic performance. Keywords: MOF Precursor; NiFe-oxide; Nanocubes; Nanoparticles; Electrocatalysis; Water splitting.

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1. Introduction Electrochemical water splitting is one of the principle methods to garner hydrogen fuel as a clean and abundant energy resource with its high energy storage density as well as zero carbon emission.1 Combining two half-reactions namely hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), overall water splitting theoretically needs a minimum voltage of 1.23 V, however a much higher cell potential of 1.8–2.0 V is generally required for the commercial electrolyzers because of its uphill reaction.2 Although noble metal such as Pt (for HER),3 and IrO2, RuO2 (for OER),4 are used as benchmark electrocatalysts, their high-cost and lowabundance hinder large-scale commercial applications. The highly researched low-cost and earth abundant electrocatalysts include transition metal oxides,5 hydroxides,6 and chalcogenides,7 as well as non-metals,8 for OER and carbides,9 sulfides,10,11 phosphides,11 and nitrides,12,13 for HER. Among them transition metal based oxides demonstrate superior performance for both HER and OER.15-17 The extensively studied Ni-based catalysts in fact suffer from their weak electronic conductivity and instability.18 The conductivity issue can be resolved by incorporating carbon nanostructures such as carbon nanotubes,19 and graphene,20 or by doping certain metals. Few recent studies demonstrate that the presence of Fe in Ni-based electrocatalysts can enhance faster electron transfer between the electrode and electrolyte, and facilitate the formation of conducting Ni-oxyhydroxide from insulating Ni(OH)2 thereby oxidizing water at reduced overpotential.21 Considering the growing popularity of mixed Ni-Fe oxides and hydroxides as efficient water splitting electrocatalysts,22 their structure and morphology needs fine tuning through chemical synthesis routes and nanostructure design. In order to sustain commercially viable overall water splitting with these earth-abundant catalysts, both OER and HER should be operated at lower overpotentials in the same acidic or 2 ACS Paragon Plus Environment

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basic electrolyte environment and its efficacy can be improved by utilizing a bifunctional OER and HER electrocatalyst. For hydrogen mass production, alkaline water electrolysis has already found widespread commercialization,23 while designing bifunctional electrocatalyst operating under strongly alkaline conditions is seldom achieved and still remains a major challenge. Some of the recent success in bifunctional catalysts includes Co-P-derived film,24 NiFe layered double hydroxide (LDH) on Ni-foam,2 Co-oxide/N-doped carbon hybrids,25 and porous cobalt phosphide/cobalt phosphate thin film,26 penroseite (Ni,Co)Se2 nanocages on 3D graphene,27 Ni5Fe LDH@NF,28 N-Ni3S2/NF,29 NiFe-P nanocubes etc.30 In order to enhance the exposure of active sites to the electrolyte, catalysts with porous morphologies can be rendered by using precursor templates with high surface area and well-organized pore structure, such as metal– organic frameworks (MOFs), which have already shown significant potential towards applications namely gas storage, drug delivery, sensing and catalysis.31 Moreover, the shape and size of the catalyst nanostructures also play an essential role in enhancing the electrochemically active surface area (ECSA) to facilitate mass diffusion.32,33 In this respect, nanostructured cubic morphologies with exposed edges and vertices containing catalytically active sites are particularly attractive for OER and HER as well as battery applications (Table S1, supporting information).34-36 However nanocubes of a suitable combination of earth-abundant materials were rarely explored as bifunctional electrocatalysts for overall water splitting.17 Inspired by the previous success of Ni-Fe-oxide electrocatalysts and this unique nanocube morphology that can be prepared from mesoporous MOF precursors, we have used NiFe Prussian blue analog (NiFe-PBA) MOF to develop mesoporous NiFe-oxide nanocube (NiFe-NC) bifunctional stable electrocatalyst that can split water at a cell voltage of 1.67 V to achieve 10 mA cm-2 in an alkaline medium. The NiFe PBA cubes were transformed into NiFe-



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NCs by thermal treatment in air.37 The merit of this MOF-derived porous NiFe-NCs consisting of a composite of NiO, γ-Fe2O3 and α-Fe2O3 is its narrow NC-size distribution, exposed catalytically active edges and vertices, and availability of both internal and external surfaces in the three-dimensional architecture for enhanced mass diffusion, improved activity and reasonable durability. 2. Results and Discussion 2.1. Formation of NiFe-NCs and NiFe-NPs The formation of porous NiFe-NCs and NiFe-NPs is schematically illustrated in Fig. 1. Firstly, NiFe-PBA was synthesized via a modified precipitation method by mixing a tri-sodium citrate solution of Ni(II) ions with a solution of potassium hexacyanoferrate. Sodium citrate dihydrate acts as a chelating ligand and coordinates with Ni(II) ions to control the nucleation rate and crystal growth. In the absence of citrate ions, the rate of nucleation is faster resulting in an aggregated morphology.38 The field emission scanning electron microscope (FESEM) image in Fig. 1 reveals the smooth surfaced uniformly sized NiFe-PBA cubes having an average side length of ~320 nm. When NiFe-PBA was calcined at 350oC in air, rough surfaced ~200 nm NiFe-NCs were obtained. The reduction in size is due to the decomposition of –CN groups starting at ~240oC (Fig. S1). The heat induced decomposition of –C≡N groups in NiFe-PBA is further confirmed by fourier transform infrared (FTIR) spectroscopy (Fig. S2). The characteristic ν(FeII-CN-NiII) and ν(FeIII-CN-NiII) stretching peaks at 2099 and 2168 cm-1, respectively in NiFe-PBA (Ni3[Fe(CN)6]2),39 are absent in the IR spectrum of NiFe-NCs. The FTIR spectra of NiFe-NCs and NiFe-NPs instead show the bands at 3442, 1636, 1382, 678 and 552 cm-1 corresponding to the hydroxyl stretching vibration of surface adsorbed water molecules, their bending mode, O-C=O symmetric stretching vibration of adsorbed atmospheric CO2, and Fe-O 4 ACS Paragon Plus Environment

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stretching mode of γ-Fe2O3 and α-Fe2O3, respectively. The band at 552 cm-1 can also be attributed to Ni-O stretching vibration mode.40 Fig. 1 also illustrates the formation of NiFe-NPs of similar elemental composition to that of NiFe-NCs via conventional co-precipitation method. The thermodynamically stable spherical morphology was stabilized when the solution pH was maintained at 13. The solution processed NiFe hydroxide assemblies transform to 40-50 nm NiFe-oxide NPs after thermal treatment in air at 350oC for 2h. 2.2. Structural Analysis The energy dispersive analysis of X-rays (EDAX) analysis from 5 or more locations shows that for both NiFe-NCs and NiFe-NPs, the molar ratio of Ni to Fe is close to 3:2 (Fig. S3), specifically 1.43 and 1.46, respectively (Table S2). The homogeneous elemental distribution is ascertained from EDAX analysis on different NCs (Fig. S3), mapping and line scan analyses (Fig. S4). While the NiFe-NPs consists of only overlapping reflections of NiO (JCPDS: 47-1049) and γ-Fe2O3 (JCPDS: 39-1346), the X-ray diffraction (XRD) pattern of NiFe-NCs consists of the face-centered cubic NiO phase,41 along with a mixture of Fe-O phases, cubic γ-Fe2O3 and rhombohedral α-Fe2O3 (JCPDS: 24-0072) (Fig. 2a). In fact, the NiFe-PBA precursor is also crystalline corresponding to the cubic Ni3[Fe(CN)6]2 PBA (JCPDS: 86-0501) (Fig. S5). The porous structure of the uniformly sized NiFe-NCs is evident from the transmission electron microscopy (TEM) images (Fig. 2b, c). The heat induced removal of gaseous molecules such as CO2 and NxOy from decomposition of –CN groups in NiFe-PBA generates the required porosity in NiFe-NCs.42,43 The composite nature of the NCs is further confirmed from the interplanar spacings (Fig. 2d) and corresponding selected area electron diffraction (SAED) patterns (Fig. 2e). The lattice spacing of 0.15 nm can be assigned to the (220) plane of the NiO phase and that



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of 0.24 nm to the (311) plane of γ-Fe2O3 phase. The α-Fe2O3 phase could not be ascertained from TEM analyses. The nitrogen adsorption/desorption results demonstrate that while NiFe-PBA precursor has a typical type-I isotherm resembling a dominant microporous structure, the NiFe-NCs are mesoporous with a type IV hysteresis loop (Fig. S6).42,44 The Brunauer–Emmett–Teller (BET) surface area of NiFe-PBA however reduces from 312.5 m2g-1 to 30.1 m2g-1 for NiFe-NCs. The NiFe-NPs are also mesoporous with a surface area of 73.6 m2g-1. The high surface area of NiFePBA is a reflection of a wide pore size distribution from 2.3 nm to 5.5 nm. On the other hand, NiFe-NCs have a mean pore diameter of 3.6 nm and the NiFe-NPs have pore distribution in the range of 3.4 and 6.5 nm. Interestingly the pore volume increases from 0.062 cm3/g for NiFe-PBA to 0.244 cm3/g for NiFe-NCs due to the decomposition of organic linkers during calcination. The pore volume of NiFe-NPs lies midway at 0.150 cm3/g. Consequent to a successful synthesis of the NiFe-NCs, their electrochemical performance towards OER, HER and overall water splitting was tested whereby their mesoporous nature is likely to facilitate mass transport suitably enhancing their performance.33,45,46 2.3. Electrochemical Water Splitting Activities 2.3.1. Electrochemical Oxygen Evolution Activity We first investigate OER activity of the electrocatalysts since OER having sluggish kinetics is a complex and energy expensive four-electron transfer process involving O-H bond breaking and O=O bond formation.47 The catalysts were deposited on the carbon fiber paper (CFP) with a fixed mass loading (2.2 mg cm-2) and their performance was investigated in alkaline 1 M KOH solution in a typical three-electrode setup. Fig. 3a demonstrates the LSV polarization curves at a scan rate of 10 mV s-1. The OER activity of NiFe-NCs is compared with 6 ACS Paragon Plus Environment

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NiFe-NPs, NiO-NPs, Fe2O3-NPs, NiFe-LDH, commercial Pt/C (40 wt%), IrO2 and bare CFP with the same mass loading under similar conditions. The Fe2O3-NPs also contain a mixture of α- and γ-phases for an effective comparison (Fig. S7). Since, the as-measured current do not reflect the intrinsic behavior of the catalysts due to ohmic resistance, an iR correction was employed for further analysis.48 The linear sweep voltammetry (LSV) polarization curves in Fig. 3a, b show that the NiFe-NCs attain a current density of 10 mA cm-2 at an overpotential of 271 mV, which is superior to the other well known catalysts (Fig. S8). The Tafel plots were derived from the LSV polarization curves using the Tafel equation (η = b log (j) + a, where η is the overpotential, j is the current density and b the tafel slope). The NiFe-NCs possess the smallest Tafel slope of 48 mV dec-1 implying the most favorable reaction kinetics for OER (Fig. 3c). The improved kinetics concurrent with the availability of active sites is confirmed from the estimation of electrochemical double-layer capacitance (Cdl), which is again directly proportional to ECSA.49 The NiFe-NCs possess Cdl of 16.3 mF cm-2 which is almost 6 times higher than that of NiFe-NPs (2.8 mF cm-2) (Fig. S9). The former has an ECSA of 65 cm2 which is higher than the spherical counterpart having 15 cm2 (Fig. S10). Even though the specific surface area of porous NiFe-NCs is lower than that of NiFe-NPs, higher Cdl and ECSA in the former attests to its better electrochemical activity. Furthermore, the turnover frequency (TOF) which reveals the intrinsic property of the catalyst (discussion S1), shows that the NiFe-NCs possess a TOF of 0.02 s-1 at an overpotential of 270 mV for OER which is larger than the NiFe-NPs (0.015 s-1). NiFeNCs also possess the least charge transfer resistance owing to fast electron transfer kinetics at the interface between electrode and electrolyte, as evidenced from the Nyquist plots in electrochemical impendence spectroscopy (EIS) measurements (Fig. S11).50 In order to estimate the contribution of the unique cubic morphology of NiFe-NCs towards OER, the catalytic current



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of both NiFe-NCs and NiFe-NPs are normalized by their respective ECSA and BET surface area (Fig. S12). The normalized LSV plots further demonstrate a better catalytic activity for NiFeNCs over the spherical counterpart (Fig. S13). Chronoamperometric test was performed to check the durability of the NiFe-NC electrocatalysts. The NiFe-NCs show excellent stability for 18 h at an overpotential of 305 mV, and the LSV plot taken after stability test shows almost similar activity with slight decrease in the current density (Fig. 3d). After the stability test, a new oxidation peak appears at around 1.46 V vs RHE due to the transformation of Ni-hydroxide (NiII) to Ni-oxyhydroxide (NiIII) formed on the surface of the catalyst due to surface oxidation.51 Post OER durability test in 1 M KOH, the catalyst displays the same diffraction peaks as those of the original NiFe-NCs/CFP, confirming the long-term phase stability of the NiFe-NCs under anodic conditions (Fig. S14). Two new reflections corresponding to the formation of α-FeOOH (JCPDS: 81-0464) and β-Ni(OH)2 (JCPDS: 14-0117) are also observed corroborating the electrochemical results. Moreover, the cubic morphology is retained after the 18 h durability test (Fig. 3e). Along with a high catalytic activity, high energy conversion efficiency is another benchmarking parameter for an efficient electrocatalyst. The Faradaic efficiency was measured by collecting the actual amount of oxygen gas catalyzed by NiFe-NCs at a constant potential of 0.62 V vs Ag/AgCl, by using a water displacement method. The NiFe-NCs shows a Faradaic efficiency of 96.4% (Fig. 3f), suggesting that almost all the charge is consumed for OER without allied parasitic reactions. In fact, the NiFe-NPs also exhibit similar durability at an overpotential of 369 mV and OER Faradaic efficiency of 100.4% at a constant potential of 0.6 V vs Ag/AgCl (Fig. S15). 2.3.2. Electrochemical Hydrogen Evolution Activity


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The HER activity of the bifunctional NiFe-NCs is compared with commercial Pt/C (40 wt%), NiFe-NPs, Fe2O3-NPs, NiO-NPs, NiFe-LDH and bare CFP under similar conditions. As expected, Pt/C shows the best HER activity with an extremely low overpotential of 20 mV at 10 mA cm-2. NiFe-NCs albeit requires a low overpotential of 197 mV to reach a current density of 10 mA cm-2 (Fig. 4a, b), thus outperforming the others (Fig. S8). While Pt/C shows the smallest Tafel slope of 58 mV dec-1 indicating HER taking place on Pt/C following a Volmer-Heyrovsky mechanism with electrochemical desorption as the rate-determining step,52 NiFe-NCs also show a small Tafel slope of 130 mV dec-1. However on NiFe-NCs HER follows Volmer-Heyrovsky mechanism where Volmer step is the rate-determining step.53 Both Pt/C and NiFe-NCs possess much lower charge-transfer resistance, demonstrating faster electron-transfer rates (Fig. S11). The performance of NiFe-NCs surpasses the other catalysts giving an insight into their superior HER kinetics (Fig. 4c). Also the NiFe-NCs possess a higher TOF of 0.4 s-1 at an overpotential of 401 mV for HER than NiFe-NPs with 0.2 s-1 (discussion S1). The ECSA and BET surface area normalized polarization curves also suggest the role of unique cubic morphology of NiFe-NCs towards HER (Fig. S12 and S13). In the chronoamperometric tests NiFe-NCs demonstrate 18 h durability at an overpotential of 238 mV (Fig. 4d). The overlapping LSVs before and after the stability test (Fig. 4d inset) highlight the retained active sites whereby their morphology remains intact (Fig. 4e). XRD measurement of the catalyst after HER durability test in 1 M KOH indicates excellent long-term crystal phase stability of NiFe-NCs/CFP in the cathodic electrochemical reactions (Fig. S14). It has been recently pointed out that when Pt is used as the counter electrode during HER, Pt may dissolve in the electrolyte and redeposit on the working electrode, which in turn contributes to the HER activity.54 In our case, as a precautionary measure when graphite rod is 9 ACS Paragon Plus Environment


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used as the counter electrode along with NiFe-NCs/CFP as working electrode, the LSV polarization curve is found to be quite similar to when Pt is used, except a slight increase in overpotential at higher current densities (Fig. S16), strongly suggesting that the HER performance of NiFe-NCs indeed reflects the intrinsically high catalytic activity of NiFe-NCs instead of any possible Pt contamination. HER Faradaic efficiency of the NiFe-NCs was measured by using the water displacement method and collecting the amount of H2 (g) at a constant potential of -1.7 V vs Ag/AgCl. The NiFe-NCs show a Faradaic efficiency of 97%, indicating high energy conversion efficiency (Fig. 4f). The NiFe-NPs also demonstrate high durability at an overpotential of 394 mV and HER Faradaic efficiency of 94.5% at a constant potential of -1.7 V vs Ag/AgCl (Fig. S15). 2.3.3. Overall Water Splitting Activity Inspired by the bifunctional behavior of NiFe-NCs for both HER and OER, an electrolyzer with two electrode configuration using NiFe-NCs/CFP as both cathode and anode was constructed for overall water electrolysis. As shown in Fig. 5a the NiFe-NC || NiFe-NC electrolyzer delivers a current density of 10 mA cm-2 at a cell voltage of 1.67 V in 1 M KOH solution. The NiFe-NCs outperform the NiFe-NP || NiFe-NP which achieve the benchmark 10 mA cm-2 at a cell voltage of 1.90 V. The activity of the NiFe-NCs is comparable and even better than many Ni-based electrocatalysts reported recently (Table S3). The gas bubbles released at the respective electrodes at a cell voltage of 1.85 V is also shown in Fig. 5a (inset). Fig. 5b shows the stability of NiFe-NCs || NiFe-NCs electrolyzer for 18 h at a static potential of 1.7 V. The LSV plot taken after the stability test shows that the activity of the electrolyzer is slightly improved may be due to activation of the catalyst (Fig. 5b inset).


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A careful selection of the NC-precursor i.e. cubic Ni3[Fe(CN)6]2 (NiFe-PBA) maintains the Ni:Fe molar ratio of 3:2, which results in 40 at% of Fe in the NiFe-NCs.51 α/γ-Fe2O3 phases serve as efficient current collectors and enable easy electron flow. 40 at% of Fe gives a certain wt% of the α/γ-Fe2O3 phases suitable for (i) increasing the electrical conductivity and electrode capacity, (ii) lowering the OER and HER overpotential by providing efficient electron transfer pathways, and (iii) suppressing the NiII to NiIII oxidation.21,51 At this Fe:Ni composition, the oxidation wave of the Ni(OH)2/NiOOH redox couple shifts to higher potentials during OER and the peak area decreases. As a result the NiII/NiIII peak is not observed in the LSV polarization curves in Fig. 3a. Since Fe already exists in the form of α/γ-Fe2O3, no Fe0 to FeII/FeIII oxidation peak is observed and if they were present, the peak would occur beyond the potential window of measurement between -0.5 and -1.2 V vs Hg/HgO (1 M). Overall, the Ni:Fe molar ratio of 3:2 is ideal to enable the OER and HER electrocatalytic processes for water splitting with NiII as the active sites and a right proportion of FeIII to enhance the electrode kinetics. 2.3.4. Contribution of the NC Geometry towards High Catalytic Activity From the above electrochemical tests it is evident that the performance of NiFe-NCs is superior to the NiFe-NPs. This can be attributed to the presence of exposed defect-rich NPs at the edges and vertices in the NCs.55 In that respect it is also pertinent to probe the activity of NPs present on these sites in comparison to the atoms on the NC faces. In order to verify the sitespecific activity, the edges and vertices of the NiFe-PBA cubes were etched with ammonia at different time intervals of 5, 10 and 15 min, followed by thermal treatment in air at 350°C for 2 h to obtain the etched NiFe-NCs. Since the edges and vertices have higher surface energy due to a high density of defect sites than the cube faces,37 etching of the NiFe-PBA cubes initiates from



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the vertices followed by the edges and body diagonal. With increased etching, the NiFe-PBA cubes collapse into a disc-like morphology, resembling the cube faces (Fig. 6a). After thermal treatment the etched nanostructures retain the shape of their precursor. The electrocatalytic activities of the etched NiFe-NCs are compared with the NiFe-NCs for both HER and OER (Fig. 6b, c) and the activities are found to decline with increasingly etched structures. The OER overpotential required for NiFe-NCs, etched NiFe-NCs-5 min, NiFe-NCs-10 min and NiFe-NCs15 min catalysts is 271, 304, 353 and 364 mV, respectively to attain 10 mA cm-2. Those for HER are 197, 256, 303 and 358 mV, respectively. Thus the necessity of exposed active sites at the NC edges and vertices facilitating more electrolyte interaction for electrocatalytic water splitting is well validated. Also, the TEM images in Fig. S17 suggests that along with the removal of highly exposed NPs from the corners and edges of the NCs, the etched NiFe-NCs get converted into hollow NCs (etched NiFe-NC-5 and 10 min) and finally collapsing into disc-like morphology resembling the faces of the NC after 15 min. Although ammonia etching results in a hollow morphology of etched NCs, but the corresponding nitrogen adsorption/desorption isotherms reveal a wide pore size distribution along with almost similar BET surface area values of 30.1, 28.6, 32.2 and 26.5 m2 g-1 for NiFe-NCs, etched NiFe-NC-5 min, etched NiFe-NC-10 min and etched NiFe-NC-15 min, respectively (Fig. S17), thereby eliminating any subtle role of specific surface area and porosity in the diminishing electrocatalytic performance of etched NiFe-NCs. Also the LSV polarization curves for OER and HER of etched NiFe-NCs normalized by their respective BET surface area (Fig. S18) follows the same trend as Fig. 6b, c indicating the dependence of the catalytic activity on the availability of defect-rich sites at the corners and edges of the nanocubes. Therefore, the abundant resource of exposed NPs at the edges and


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vertices of NCs with electrochemically active sites is mainly responsible for their excellent performance as bifunctional catalyst in water splitting. 3. Conclusions In summary, this work demonstrates a highly efficient bifunctional nanocatalyst with cubic morphology for overall water splitting. The ~200 nm NiFe-NCs have a Ni:Fe molar ratio of 3:2 and is a composite of NiO, γ-Fe2O3 and α-Fe2O3. The mesoporous NiFe-NCs were derived from NiFe-PBA (MOF) and with its cubic morphology the NCs outperform the electrocatalytic activity of spherical NiFe-NPs. The bifunctional NiFe-NCs catalyzes the overall water splitting reaction delivering a current density of 10 mA cm-2 at a cell voltage of 1.67 V in alkaline medium with reasonable stability. The improved catalytic activity of NiFe-NCs can be attributed to the following factors: (i) Employing a MOF with high surface area as the precursor which helps in preventing the aggregation of primary NPs, thus providing more exposed active sites and sufficient specific surface area suitable for electrocatalysis. (ii) Mesoporous 3-dimensional structure with broad pore size distribution facilitating fast diffusion of fresh reactants towards the active sites as well as diffusion of gaseous products away from the active sites. (iii) Exposure of electrochemically active sites at the edges and corners of the NCs which also leads to higher ECSA and overall catalytic performance. Our findings although are restricted to electrochemical water splitting, these NCs have the potential to be employed in other energy conversion and storage applications. 4. Experimental Section 4.1. Materials



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Nickel (II) nitrate hexahydrate Extra Pure (Ni(NO3)2.6H2O; Merck, ≥97 %), potassium hexacyanoferrate (III) (K3Fe(CN)6; Sigma Aldrich, ≥99%), iron (III) nitrate nonahydrate (Fe(NO3)3.9H2O;

Sigma

Aldrich,

≥98%),

tri-sodium

citrate

dihydrate

Purified

(C6H5Na3O7.2H2O; Merck, ≥99 %), sodium hydroxide pellets (NaOH; Merck, ≥97 %), ammonia solution (Merck, ≥ 25 %), potassium hydroxide pellets (KOH; Merck, ≥ 85 %), ethanol (C2H5OH; Merck ≥ 99.9 %), commercial Pt/C (40 wt%, Sigma Aldrich), toray carbon fiber paper (CFP; Alfa Aesar) and nafion perfluorinated resin solution (5 wt%, Sigma Aldrich) were used without further purification.

4.2. Methods

4.2.1. Synthesis of NiFe prussian blue analog (NiFe-PBA) cubes

Uniform NiFe-PBA nanocubes were synthesized following a modified precipitation method.37 Nickel nitrate (1.5 mmol) and tri-sodium citrate dihydrate (2.25 mmol) were dispersed in 50 ml of distilled water to form solution A (Fig. 1). Potassium hexacyanoferrate (1 mmol) was dispersed in 50 ml of distilled water to form solution B. Solutions A and B were mixed under vigorous stirring for 10 min. This mixture was kept undisturbed at room temperature for 7 days. Thereafter the product was collected by centrifugation, washed with ethanol and water and dried at 60°C overnight.

4.2.2. Synthesis of porous NiFe-oxide nanocubes (NiFe-NCs)

For the synthesis of porous NiFe-NCs, as-prepared NiFe-PBA nanocubes were calcined in air inside a tube furnace at 350°C for 2 h at a heating rate of 2.5 °C min-1.


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4.2.3. Synthesis of NiFe-oxide nanoparticles (NiFe-NPs)

NiFe-NPs with similar chemical composition as that of NiFe-NCs were synthesized using a co-precipitation method. Ni(NO3)2.6H2O (1.5 mmol) and Fe(NO3)3.9H2O (1 mmol) were dissolved in 100 ml of distilled water. 2 M NaOH was added drop by drop, till pH 13. The solution was warmed at 80°C for 60 min and the resultant precipitate was collected by centrifugation, washed several times with distilled water and ethanol and finally dried at 60°C overnight. The dried precipitate was calcined in air at 350°C for 2 h inside a tube furnace at a heating rate of 2.5 °C min-1 to obtain NiFe-NPs.

4.2.4. Synthesis of NiO NPs and Fe2O3 NPs NiO and Fe2O3-NPs were synthesized by following the same procedure for NiFe-NPs except Ni(NO3)2.6H2O and Fe(NO3)3.9H2O were separately used as precursors.

4.2.5. Synthesis of etched NiFe-NCs

To obtain NiFe-NCs with etched corners and edges, 20 mg as-prepared NiFe-PBA nanocubes was dispersed in 10 ml of ethanol. 20 ml of distilled water containing 2.5 ml of ammonia solution was added to the above solution containing NiFe-PBA nanocubes and stirred for 5, 10, and 15 min at room temperature. The three differently stirred precipitates were collected by centrifugation and washed with ethanol and water. After drying at 60°C overnight, the precipitates were calcined in air at 350 °C for 2 h at a heating rate of 2.5 °C min-1.

4.2.6. Synthesis of IrO2



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The synthesis protocol of IrO2 was adopted from a literature report.56 Briefly, K2IrCl6 (0.2 mmol) was added to 50 ml aqueous solution of 0.16 g of C6H5Na3O7.2H2O (0.63 mmol). The red-brown solution was adjusted to pH 7.5 using NaOH solution (0.25 M) and heated to 95°C in an oil bath under constant stirring. After heating for 30 min, the solution was cooled to room temperature and NaOH solution was added again to adjust pH 7.5. The addition of NaOH solution at room temperature, followed by heating at 95 °C for 30 min, was repeated until the pH was stabilized at 7.5. The IrOx colloidal solution was precipitated by centrifugation and dried overnight in a desiccator. Finally, the dried powder was calcined in a box-furnace at 400oC for 30 min to obtain the oxide.

4.2.7. Synthesis of NiFe-LDH

NiFe-LDH was synthesized by following a general protocol adopted from the literature.57 Briefly, Ni(NO3)2.6H2O (0.6 mmol), Fe(NO3)3.9H2O (0.4 mmol), NH4F (5 mmol), and urea (30 mmol) were dissolved in 30 mL of distilled water. Then the solution was transferred into a Teflon-lined stainless autoclave and the autoclave was heated to 180°C for 4 h. After cooling down to room temperature, the green precipitate was collected, cleaned by distilled water and ethanol for several times and dried at 60°C for 12 h.

4.3. Characterization

The FESEM images were obtained using a Carl Zeiss SUPRA 55VP FESEM. EDAX spectra were performed in an Oxford Instruments X-Max with the INCA software coupled to the FESEM. TEM images were recorded with the DST-FIST facility, IISER Kolkata, JEOL, JEM-2100F using a 200 kV electron source. The FTIR spectroscopy studies were performed


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with a Perkin Elmer spectrum RX1 with KBr pellets. The powder XRD measurements were performed with a Rigaku (mini flex II, Japan) powder X-ray diffractometer having Cu Kα = 1.54059 Å radiation. The surface area and porosity measurements were performed with a Micromeritics Gemini VII surface area analyzer and N2 adsorption-desorption isotherms were reported by BJH (Barrett–Joyner–Halenda) surface/volume mesopore analysis. All the samples were degassed at 150 °C for 6 h. The specific surface area was determined according to the BET method.

4.4. Electrochemical measurements

All the electrochemical measurements were carried out using an electrochemical workstation (CH Instruments, Model CHI604D) and the plots were analyzed with inbuilt software in the electrochemical workstation. The electrochemical tests for OER, HER and electrochemical double layer capacitance were measured in a conventional three-electrode electrochemical cell and overall water splitting was performed in a two-electrode system in 1 M KOH. The commercial CFP was decorated with catalyst ink and used as a working electrode. A platinum wire and Ag/AgCl (3M KCl) served as counter electrode and reference electrode, respectively. The catalyst ink was prepared by dispersing 6 mg of catalyst into 500 µl of ethanol containing 10 µl of 5% Nafion solution and sonicated for 60 min. A certain quantity of the catalyst ink was drop-casted onto CFP and left to dry in air (loading amount: 2.2 mg/cm2). Then the working electrode was coated with 10 µl of 0.5 % Nafion solution in ethanol and dried at 60 °C overnight. Before recording the electrochemical activity of the catalyst, all the working electrodes were saturated at higher oxidation and reduction potentials for few seconds and also using cyclic voltammetry (CV) scans at a scan rate of 100 mV s-1. LSV measurements were



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recorded at a scan rate of 10 mV s-1 in order to minimize the capacitive current,22 and this scan rate is slow enough to ensure steady-state behavior at the electrode surface.49 EIS studies were performed to compare the charge transfer resistance (RCT) in the faradaic region since RCT is inversely proportional to the charge transfer. All the potentials reported were converted from versus Ag/AgCl to RHE using the equation:58 E(RHE) = E(Ag/AgCl) + E0(Ag/AgCl) + 0.059*pH; in order to exclude the influence of pH, reference electrodes and internal solutions e.g. KCl. To calculate the current density, the working surface area was calculated on a single side. The stability of the catalyst was tested by bulk electrolysis (chronoamperometry). To measure the faradaic efficiency, the actual amount of gas (oxygen and hydrogen) produced was measured using the water displacement method in an air-tight vessel.59 ECSA was obtained from the Cdl values by collecting CVs at various scan rates (5, 10, 15 and 20 mV s-1) in a non-faradaic region (1.14 V to 1.228 V versus RHE) since ECSA is proportional to the Cdl value in the same material according to the following equation: Cdl = Ic/ν, where Cdl, Ic and ν are the double layer capacitance (Fcm-2) of the electroactive materials, charging current (mAcm-2) and scan rate (mVs-1), respectively. All the potentials in three-electrode HER and OER measurements were 85% iR-corrected with respect to the ohmic resistance of the solution unless specified and calibrated to RHE based on the equation: E(RHE) = E(Ag/AgCl) + E0(Ag/AgCl) + 0.059*pH – 85%iRs. Acknowledgements AK thanks DST-INSPIRE for his fellowship. The financial support from DST-SERB under sanction no. EMR/2016/001703 is duly acknowledged. Electronic Supplementary Information Available Comparative literature review of electrocatalysts with cubic morphology; TGA analysis; FTIR spectra; EDAX spectra and elemental mapping; XRD pattern of NiFe-PBA, NiO NPs and Fe2O3


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NPs; N2 adsorption-desorption plots; Comparative LSV plots; ECSA determination; TOF calculations; Nyquist plots; Normalized LSV plots; XRD patterns of catalysts post OER/HER; OER and HER stability tests of NiFe-NPs; LSV plots with different counter electrodes; Comparison with reported Ni-based electrocatalysts; TEM, surface area and normalized LSV plots of etched nanostructures. References (1) Zou, X.; Zhang, Y. Noble Metal-free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148−5180. (2) Luo, J.; Im, J. H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N. G.; Tilley, S. D.; Fan, H. J. F.; Grätzel, M. Water Photolysis at 12.3% Efficiency via Perovskite Photovoltaics and Earth-abundant Catalysts. Science 2014, 345, 1593-1596. (3) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode over the Wide Range of pH 0−14. J. Am. Chem. Soc. 2014, 136, 7587−7590. (4) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399−404. (5) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Metal−Organic Framework Derived Hybrid Co3O4-Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136, 13925−13931. (6) Song, F.; Hu, X. Ultrathin Cobalt−Manganese Layered Double Hydroxide is an Efficient Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2014, 136, 16481−16484. (7) Liu, Y.; Cheng, H.; Lyu, M.; Fan, S.; Liu, Q.; Zhang, W.; Zhi, Y.; Wang, C.; Xiao, C.; Wei, S.; Ye, B.; Xie, Y. Low Overpotential in Vacancy-Rich Ultrathin CoSe2Nanosheets for Water Oxidation. J. Am. Chem. Soc. 2014, 136, 15670−15675. (8) Datta, A.; Kapri, S.; Bhattacharyya, S. Carbon Dots with Tunable Concentrations of Trapped Anti-oxidant as an Efficient Metal-free Catalyst for Electrochemical Water Oxidation. J. Mater. Chem. A 2016, 4, 14614-14624. (9) Liao, L.; Wang, S.; Xiao, J.; Bian, X.; Zhang, Y.; Scanlon, M. D.; Hu, X.; Tang, Y.; Liu, B.; Girault, H. H. A Nanoporous Molybdenum Carbide Nanowire as an Electrocatalyst for Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 387 –392.



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Figure 1: Schematic illustration of the formation of NiFe-PBA derived porous NiFe-NCs and NiFe-NPs via co-precipitation method.



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Figure 2: (a) XRD pattern of NiFe-NCs and NiFe-NPs. (b) Low and (c) high magnification TEM images of NiFe-NCs. (d) Lattice spacing showing the composite nature of the NCs. (e) SAED pattern from the region shown in (d).


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Figure 3: (a) LSV polarization curves for OER and (b) the required overpotentials to achieve 10 mA cm-2. (c) Tafel plots with the corresponding Tafel slopes for the OER process. (d) Chronoamperometric durability test at a constant overpotential of 305 mV. Inset shows the LSV curves before and after the durability test. (e) FESEM image of NiFe-NCs after the OER durability test. (f) Faradaic efficiency measurement of NiFe-NCs showing the theoretically calculated and experimentally measured O2 gas with time, at 0.62 V vs Ag/AgCl.



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Figure 4: (a) LSV polarization curves for HER and (b) the required overpotentials to achieve 10 mA cm-2. (c) Tafel plots with the corresponding Tafel slopes for the HER process. (d) Chronoamperometric durability test at a constant overpotential of 238 mV. Inset shows the LSV curves before and after the durability test. (e) FESEM image of NiFe-NCs after the HER durability test. (f) Faradaic efficiency measurement of NiFe-NCs showing the theoretically calculated and experimentally measured H2 gas with time, at -1.7 V vs Ag/AgCl.


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Figure 5: (a) Polarization curves of the two-electrode setup with bifunctional catalysts for overall water-splitting in 1 M KOH (iR un-corrected). Inset shows the optical image of O2 and H2 bubbles generated for NiFe-NCs at a cell voltage of 1.85 V. (b) Chronoamperometry of water electrolysis at a cell voltage of 1.7 V for NiFe-NCs. Inset shows the LSV curves before and after the durability test.



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Figure 6: (a) FESEM images of the 5, 10 and 15 min etched NiFe-PBA NCs and the corresponding etched NiFe-NCs obtained by heat treatment. LSV polarization curves for (b) OER and (c) HER, showing the decreasing activity of the nanostructures with increasing time of ammonia etching.


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