Remarkable Bifunctional Oxygen and Hydrogen Evolution

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Remarkable bifunctional oxygen and hydrogen evolution electrocatalytic activities with trace level Fe-doping in Niand Co-layered double hydroxides for overall water splitting Gaddam Rajeshkhanna, Thangjam Ibomcha Singh, Nam Hoon Kim, and Joong-Hee Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16425 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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ACS Applied Materials & Interfaces

Remarkable bifunctional oxygen and hydrogen evolution electrocatalytic activities with trace level Fe-doping in Ni- and Co-layered double hydroxides for overall water splitting G. Rajeshkhanna,† Thangjam Ibomcha Singh,† Nam Hoon Kim,*† and Joong Hee Lee**†‡ †Advanced

Materials Institute for BIN Convergence Technology (BK21 plus Global Program), Department of BIN Convergence Technology and ‡Carbon Composite Research Centre Department of PolymerNano Science and Technology, Chonbuk National University Jeonju, Jeonbuk 54896, Republic of Korea. ABSTRACT Large-scale H2 production from water by electrochemical water splitting is mainly limited by the sluggish kinetics of the non-precious based anode catalysts for oxygen evolution reaction (OER). Here, we report layer-by-layer in situ growth of low-level Fe-doped Ni-layered double hydroxide (Ni1-xFex-LDH), and Co-layered double hydroxide (Co1-xFex-LDH), respectively, with 3D microflower and 1D nanopaddy-like morphologies on Ni foam, by a one-step eco-friendly hydrothermal route. In this work, an interesting finding is that both Ni1-xFex-LDH and Co1-xFexLDH materials are very active and efficient for OER as well as hydrogen evolution reaction (HER) catalytic activities in alkaline medium. The electrochemical studies demonstrate that Co1xFex-LDH

material exhibits very low OER and HER overpotentials of 249 and 273 mV,

respectively at a high current density of 50 mA cm-2, while Ni1-xFex-LDH exhibits 297 and 319 mV. In order to study the overall water splitting performance using these electrocatalysts as anode and cathode, three types of alkaline electrolyzers are fabricated namely Co1-xFexLDH(+)ǁCo1-xFex-LDH(-), Ni1-xFex-LDH(+)ǁNi1-xFex-LDH(-) and Co1-xFex-LDH(+)ǁNi1-xFexLDH(-). These electrolyzers require only a cell potential (Ecell) of 1.60, 1.60 and 1.59 V, respectively, to drive the benchmark current density of 10 mA cm-2. Another interesting finding is that their catalytic activities are enhanced after stability tests. Systematic analyses are carried 1 ACS Paragon Plus Environment

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out on both the electrodes after all electrocatalytic activity studies. The developed three types of electrolyzers are very efficient to produce H2, cost-effective, offers no complications in synthesis of materials and fabrication of electrolyzers, which can greatly enable the realization of clean renewable energy infrastructure. KEYWORDS: Ni1-xFex-LDH, Co1-xFex-LDH, bifunctional electrocatalysts, oxygen evolution reaction, hydrogen evolution reaction, overall water splitting 1. INTRODUCTION In the coming years, research and development on clean energy is becoming of foremost importance due to rapid climate change and the depletion of fossil fuels supplies. Advanced technologies for clean energy conversion include fuel cells, CO2-to-fuel conversion, water electrolysis, and metal-air batteries.1–9 In the future, these devices will serve as the main components for sustainable energy utilization and storage. Hydrogen production via water electrolysis results in electricity production with no greenhouse gas emission, which is clean, renewable, and secure. The surplus electricity production from solar and wind can be utilized for producing H2 through electrolysis, and it can be used as feedstock for fuel cells to generate electricity.6–9 Electrolysis is the process of splitting water into hydrogen and oxygen at cathode and anode, respectively, using electricity.7 The free energy change for the splitting of 1 molecule of H2O to 1 molecule of H2 and 0.5 molecule of O2 (H2O (l) → H2 (g) + 0.5 O2 (g)) under standard conditions is ΔG = +237.2 kJ mol-1.10 Therefore, the thermodynamic standard potential 0 0 0 ( E cell ) for the water electrolysis cell is 1.23 V at 25 °C ( E cell = -G /nF , where n=2, F=96485 C

mol-1), irrespective of the reaction media (neutral/acid/alkaline).5,10,11 In reality, excess potential is required to generate H2 and O2 from H2O, in order to overcome the intrinsic activation barriers 2 ACS Paragon Plus Environment

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existent on both anode (a) and cathode (c), as well as other resistances (other), for instance solution resistance and contact resistance. The amount of excess potential required is termed as overpotential (), which is essential to drive water splitting. Therefore, the practical operational voltage (Eop) for water splitting is defined as Eop = 1.23 V + a + c + other.5–7 In the process of electrolysis, the HER is severely controlled by the reaction kinetics of the OER.12 Therefore, an efficient OER catalyst with lower overpotential can improve the energy efficiency of overall water splitting. At present, two technologies are available in the market, namely alkaline and proton exchange membrane (PEM) electrolyzers. Alkaline electrolyzers are economical in terms of investment (they mostly use nickel and iron-based catalysts), but they are less efficient. While PEM electrolyzers are more expensive (they use platinum-based metal catalysts), but they are more efficient, and can function at higher current densities.8 For a well-designed electrolysis cell, the largest overpotential is the reaction overpotential for OER at the anode, which is a four+ 0 electron process ( 2H 2 O(l)  O 2 (g) + 4 H (aq) + 4e , E = +1.23 V ).5,11 Therefore, the main

current problem is how to effectively catalyze OER on the electrode surface to accomplish as low overpotential and as high current density as possible. To facilitate this reaction, precious metal oxides (RuO2 and IrO2) are regarded as state-of-the-art material, and they are stable at highly oxidizing conditions.2,13 However, to be economical, an inexpensive and efficient electrocatalyst for OER would be a great breakthrough, and it is the focusing topic of the current research. The HER at cathode can be electrocatalyzed by platinum with almost no overpotential, + 0 which is a two-electron process ( 2H (aq) + 2e  H 2 (g), E = 0.00 V ).5,11 When less expensive

non-Pt-based materials are used as cathode, large overpotentials will appear, and also, they are

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relatively less efficient.2,5,6 Alternative to the precious metal-based catalysts, great research efforts have been devoted to explore cost-effective OER, as well as HER catalysts; they include transition metal alloys, metal hydroxides, oxides, sulfides, carbides, selenides, phosphides, nitrides, oxyhydroxides, oxysulfides, borides, hydroxide carbonates, hydroxyl phosphates, and carbon-based materials.2,5,6,14–21 Because of the thermodynamic convenience, the HER and OER catalysts show higher activity in strong acidic and basic milieu, separately. However, for overall water splitting, pairing two types of catalysts, either in an integrated PEM, or alkaline electrolyzers with high efficiency and stability, is quite difficult, due to the discrepancy of the electrolyte pH.22 However, strong alkaline media is much less corrosive than strong acidic condition.22 Therefore, intense attention has been focused on the development of efficient and durable bifunctional electrocatalysts for both HER and OER in the same electrolyte, which is a great breakthrough. In this regard, first-row transition metal-based materials (hydroxides, oxides, oxyhydroxides) are of particular interest, because of their low cost, abundance, and easy synthesis. In addition, they exhibit rich chemistries, variable oxidation states, favourable phase changes during the electrochemical processes, and they can be easily tuned and synthesized at large-scale for targeted applications. They are relatively stable catalysts in alkaline medium in the pH range (7–14) for both HER and OER activities.5,15,22,23 Recent studies show that iron-based metal hydroxide and oxyhydroxides are the most active catalysts for OER, as well as HER.23–26 In particular, Ni-, Co-, and Fe-based LDH materials are recognized as an emerging family of efficient electrocatalysts for OER in alkaline media, due to the most-optimal M−O bond strength (OER intermediates, e.g., M−OH, M−O, M−OOH, M−OO).23–26 Very recently, Yin et al. studied the OER activity of NiFe-LDH/C (Ni0.67Fe0.33/C) nanohybrid, which shows 210 mV overpotential at a benchmark current density of 10 mA cm-2;24 4 ACS Paragon Plus Environment

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Liu et al. reported the OER activity of CoFe-LDHs, which shows 232 mV overpotential;25 However, these studies are limited to half-cell reactions, the study of overall water splitting activity is the most important consideration for knowing the efficiency of the actual catalyst. Lately, many attempts have been made to develop highly active and efficient durable catalysts for water electrolysis that show efficient bifunctional properties (OER and HER) with lower overpotentials and improved reaction rates.27,28 However, the potential required for overall water splitting to drive the current density of 10 mA cm-2 is still high; nevertheless, they opened ways to develop efficient bifunctional electrocatalysts for electrochemical water splitting. In recent days, although great progress has been made in the direction of the development of bifunctional materials, quite a few problems still remain in the system. These include: (i) low efficiency for overall water splitting;27,28 (ii) low durability over a wide pH range and potentials (higher oxidizing conditions), under these conditions materials will undergo destructive phase and morphological changes, leading to degradation in performance; (iii) fabrication of electrode materials using binders are destructive, in terms of decreasing conductivity and blocking electroactive sites;27,28,29 and (iv) scalability to ensure an inclusive of commercial use, usage of high-cost precursors, complications in the synthesis of nanostructured materials and the design of electrolytic cells are the main hindrances for industrial scale applications. These concerns can be resolved by facile and binder-free in situ synthesis of first-row transition metal-based materials on conductive substrate, which is advantageous and cost-effective.29,30 By considering all the aspects mentioned above, we report a facile in situ fabrication of proficient and robust nanostructured Fe doped -Ni-LDH (Ni1-xFex-LDH) and -Co-LDH (Co1xFex-LDH)

bifunctional electrocatalysts on Ni foam for OER, HER, and overall water splitting

in an alkaline milieu. The doping of Fe can provide more active sites, and enhances the 5 ACS Paragon Plus Environment


electronic and ionic conductivity.31 The direct growth of active phase on porous Ni foam avoids binder usage, and it turns out to be more expedient for good electrical conductivity and excellent durability.29,32 Electrochemical results showed that Ni1-xFex-LDH and Co1-xFex-LDH materials exhibit excellent bifunctional properties. Interestingly, the electrocatalytic activities of these materials for OER, HER, and overall water splitting enhances significantly with the stability tests, due to the constructive electronic structure and morphological changes. It is worth noting that the overall water splitting performance using these materials as anode and cathode in the same electrolyte has been tested. Three alkaline electrolysis cells are constructed with different combinations of anode and cathode catalysts, such as Co1-xFex-LDH(+)ǁCo1-xFex-LDH(-); Ni1xFex-LDH(+)ǁNi1-xFex-LDH(-);

and Co1-xFex-LDH(+)ǁNi1-xFex-LDH(-). These three electrolyzers

are produced H2 and O2 at very low potentials of 1.6, 1.6, and 1.59 V, respectively, at a current density of 10 mA cm-2. 2. RESULTS AND DISCUSSION 2.1. Structural characterization Among the wide variety of synthetic routes available to obtain small particles, a solution-based “bottom-up” approach has several advantages over a “top-down” approach. In situ assembly of nanostructures has the intrinsic ability to produce particles with a narrow distribution of both size and shape, allowing good control over the resulting product. In this regard, urea-assisted coprecipitation under hydrothermal conditions produces a wide variety of morphologies with good crystalline structures, as shown in Figure S1 of the Supporting Information (SI).29,30,32 In order to determine the phase and crystal structure of the as-synthesized materials, PXRD has been recorded. Figure 1a shows the X-ray diffraction patterns of Ni1-xFex-LDH and Co1-xFexLDH at diffraction angles (2 theta) of 11.6, 23.3, 34.3, 38.9, 59.8, and 61.1°, corresponding to 6 ACS Paragon Plus Environment

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Figure 1. (a) PXRD patterns, (b) FT-IR spectra of Ni1-xFex-LDH and Co1-xFex-LDH and (c) Schematic diagram of metal layered double hydroxide (LDH). the planes of (003), (006), (009)+(012), (015), (110), and (113). Both these diffraction patterns are characteristic of hydrotalcite-like layered hydroxide structure, which is well matched with the recently reported results and the standard JCPDS card # 38-0715.33–37 The additional peaks correspond to oxy-hydroxide phase; generally, metal oxy-hydroxide phase increases with the increase in Fe content.38 In order to avoid the high intense peaks of Ni foam the deposited material was seperated from the Ni foam under ultrasonication treatment and used for PXRD analysis. The chemical composition of LDH is represented as M(II)xM(III)x(OH)2(An)x/n × yH2O,



where M(II) is divalent cation, M(III) is trivalent cation, A is interlayer anion, n is charge on interlayer anion, and x and y are fraction constants. Figure 1c shows the typical hydrotalcite-like layered hydroxide structure; in general, LDH constituted by divalent (Ni(II)/Co(II)/Fe(II)) and trivalent (Ni(III)/Co(III)/Fe(III)) cations resides in the center of octahedrally coordinated hydroxyl groups, forming positively-charged brucite-like sheets. In order to counterbalance the positive charge on the brucite layers resulting from the existence of trivalent cations, anions occupy the interlayer spaces of two adjacent sheets.34 These anions can be exchanged with other anions present in aqueous solution. Moreover, broad and symmetric diffraction peaks show that the amorphous hydrotalcite-like double layer hydroxide phase is highly formed.37,39 In basal (00l) reflections, as l increases from (003) to (006), the intensity of the peak decreases. The intensity ratio of the (006) and (003) reflections is a measure of the interlamellar electron density. Loss of water in the interlayer spaces, associated with diminution of the electron density within the interlamellar domains, lead to less intense (006) reflections, relating to (003) diffractions.39 The less intense broad diffraction peaks are due to the poor crystallinity of the materials, as it is known from recent studies that the amorphous materials are able to show good electrocatalytic activity for OER and HER applications. Hence, these materials are expected to show high catalytic activity.34 Generally, the XRD data of -LDH materials shows non-uniform (‘‘sawtooth’’) broadening of (h0l) reflections, which are indicative of turbostratic disorder;40 however, this is not observed in the Ni1-xFex-LDH and Co1-xFex-LDH, which signifies turbostratically-ordered structure. The diffraction peaks of Ni1-xFex-LDH are significantly narrower compared to the Co1-xFex-LDH, indicating larger crystallite sizes present in the sample.40 Further, in order to find the anions (An-) present in the interlayer spacing of brucite-like layered structure, Fourier transform infrared spectroscopy (FT-IR) has been carried out on both 8 ACS Paragon Plus Environment

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samples. Figure 1b shows the FT-IR spectra of Ni1-xFex-LDH and Co1-xFex-LDH. The absorption bands at around 3392 and 3493 cm-1 are ascribed to the stretching modes of –OH in the layered hydroxyl groups and interlayer water molecules. A sharp band at 3628 cm-1 is attributed to the O–H stretching vibration of free –OH groups in brucite-like sheets, this band is not observed for Co1-xFex-LDH due to the unavailability of free –OH groups, which are bonded to the metal ions.41 An IR absorption band at 1623 cm-1 is ascribed to the bending vibrations of interlayer water molecules. The bands at around 1356 and 1502 cm-1 are credited to stretching vibrations of 2-

interlayer carbonate ( CO3 ) species. A very weak absorption band at 2184 cm-1 is ascribed to the existence of a fraction of intercalated isocyanate (NCO-).42 The bands in the region of 500–700 cm-1 are attributed to M–O, O–M–O and M–O–M (M= Ni/Co/Fe) vibrations.37,43 Intercalated carbonate and isocyanate intermediated species are generated during the hydrolysis of urea.42,44 When a fraction of divalent cations (Ni2+/Co2+/Fe2+) is isomorphously substituted by a trivalent cation, such as (Ni3+/Co3+/Fe3+), the metal hydroxide brucite layers acquire a positive charge, leading to the intercalation of anions in the interlayers spacing to maintain charge neutrality.36,43 In aqueous medium, under hydrothermal conditions, urea gradually generates ammonia by the hydrolysis, as shown in Equation 1.43 These generated NH3 molecules react with metal cations present in the solution to form metal complexes. The controlled generation of the OH ions produced from the NH3 forms octahedra with the metal cations. The Ni/Co/Fe hydroxide octahedral nuclei will self-assemble to form the infinite 2D brucite-like sheets, as shown in Figure 1c. These 2D sheets further extend, and form the metal-LDH nanosheets (Figure 1c and 2



Figure S1).40,43 However, some of the anions/molecules ( Co3 , NO3 , F  , H 2O ) generated in the



solution were expected to be retained within the interlayer space of -LDH through hydrogen bond. Additionally, the hydrothermal condition controls the structure and crystallization.29,30,32,44 𝑁𝐻2𝐶𝑂𝑁𝐻2 + 2𝐻2𝑂

𝐻𝑦𝑑𝑟𝑜𝑙𝑦𝑠𝑖𝑠

𝐻2𝐶𝑂3 + 2𝑁𝐻3

(1)

𝑀 + 6𝑁𝐻3→[𝑀(𝑁𝐻3)6](𝑀 = 𝑁𝑖2 + /𝐶𝑜2 + /𝐹𝑒2 + )

(2)

𝑁𝐻3 + 𝐻2𝑂 → 𝑁𝐻4+ + 𝑂𝐻 ―

(3)

𝑀 + 2𝑂𝐻 ― →𝑀(𝑂𝐻)2

(4)

It is known that the exposed active sites play a vital role in catalytic activities. In this regard, a rational design and synthesis of electrode material can help to improve the number of active sites, and thus enhance the electrocatalytic activity. Figure 2 shows the field emission scanning electron microscope (FESEM) images of Ni1-xFex-LDH and Co1-xFex-LDH on Ni foam, while the insets of Figure 2a and 2f show the hydrothermally deposited -LDH materials, respectively. The FESEM images of Ni1-xFex-LDH resemble chrysanthemum flower-like morphology with an average diameter of 10 m (Figure 2a, 2c and 2d), Figure 2b shows the natural chrysanthemum flower. While Co1-xFex-LDH shows a paddy field-like morphology (Figure 2f, h, and i), Figure 2g shows a natural paddy field. The microflower of Ni1-xFex-LDH material is formed with numerous two-dimensional (2D) ultrathin nanosheet-like petal building blocks, which provides highly porous three-dimensional superstructures, as clearly seen in Figure 2c. The 2D sheet-like petal structural units of Ni1-xFex-LDH vertically oriented in multi-directions produce spherical type microstructure (Figure 2c). The average thickness of petal is found to be 8 nm, which is shown in high-magnification FESEM image of the microflower in Figure 2d. The elemental mapping images of Ni1-xFex-LDH on Ni foam are shown in Figure 2e, which reveals the existence of Ni, Fe, and O elements in the 3D microflower in a specified microflower region 10 ACS Paragon Plus Environment

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(indicated by dotted circle). The FESEM images of Co1-xFex-LDH in Figure 2f and 2h show a uniformly grown 1D nanopaddy-like structure. The 1D nanostructures are expected to show high

Figure 2. (a,c,d) Low and high magnification FESEM images of Ni1-xFex-LDH (b) natural chrysanthemum flower and (e) elemental color mapping images showing the distribution of Ni, Fe and O, (f,h,i) FESEM images of Co1-xFex-LDH, (g) natural paddy field and (j) elemental color mapping images showing the distribution of Co, Fe and O. electronic and ionic conductivity. The inter-nanopaddy pores can enhance the wettability, and allow rapid liberation of the generated gas molecules at the surface of nanopaddy leaves. The



elemental mapping image of Co1-xFex-LDH on Ni foam in Figure 2j displays the presence of Co, Fe, and O elements. The reason for the resultant fascinating morphologies of 2D flakes composing the 3D microflower of Ni1-xFex-LDH and 1D nanopaddy of Co1-xFex-LDH is mainly due to the slow hydrolysis of urea at elevated temperature. The detailed formation mechanism of complex porous morphologies of -LDH materials is not clearly seen in the literature, due to the presence of various factors in the reaction mixture during the product formation, such as hydrophilic/hydrophobic interactions, Van der Waals forces, hydrogen bonding, electrostatic and dipolar fields, crystal-face attraction, intrinsic crystal contraction, and Ostwald ripening.45 The most plausible mechanism for the nucleation and growth of -LDH materials during hydrothermal synthesis is nucleation-dissolution-recrystallization-aggregation-solid phase transformation.45,46 Some of the reports from in situ studies combined with theoretical support demonstrate that the nucleation and growth take place through an aggregation-based mechanism to generate crystals with hierarchical structures. In particular, the crystal growth proceeds through the aggregation of nascent precursor units, instead of by the classical ion-by-ion mechanism.45–48 A similar type of mechanism can also be explained for the formation of the 3D microflower of Ni1-xFex-LDH and 1D nanoppaddy of Co1-xFex-LDH like superstructures. In this study, the synthesis conditions are the same for Ni1-xFex-LDH and Co1-xFex-LDH materials, except one metal salt (Nickel nitrate/Cobalt nitrate); hence, the nature of the metal cation and difference in their solubility products of hydroxides (Ni(OH)2, Ksp = 5.5 × 10-16 and Co(OH)2, Ksp = 5.9 × 10-15) played an important role during the crystal growth to obtain diverse morphologies.49 In a reaction mixture the formation of 1D, 2D, and 3D morphologies during slow reactions in the homogeneous precipitation process is ascribed to the initial formation of nascent-LDH crystallite nuclei, which begin to impinge on neighboring crystals, and assemble in a specific orientation. The 12 ACS Paragon Plus Environment

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polycrystalline particles composed of randomly oriented primary nanocrystallites are responsible for the growth of secondary particles in preferred orientations, leading to the formation of the 2D flake/petal/sheet-like structure of Ni1-xFex-LDH. It is known that temperature and time also play major roles in producing specific morphologies; under urea hydrolysis and hydrothermal conditions, nickel ions form flake/petal/sheet at low temperature and in less reaction time.50,51 When the reaction time is prolonged, numerous 2D flakes/petals/sheets begin to assemble, and form 3D flower-like morphology, as shown in Figure 2a, 2c, and 2d, in order to attain minimum total free energy. The thickness of the sheets increases with time, as sheets with bigger size and minimal surface energy are thermodynamically favored.50,51 The oriented attachment takes place in a single direction in the case of 1D nanopaddy Co1-xFex-LDH. The extrinsic crystal growth is controlled to some extent by the adsorption of surface energy modifiers (H2O and anions) on certain crystallographic planes. Hence, the initially formed -LDH nanocrystallites and their growth govern the final morphology.45–48 Note that the shape, size, and orientation of microstructure of nanocrystalline materials largely impact a material’s properties and its chemical reactivity. The materials with hierarchical 3D microflower architecture composed of 2D ultrathin sheets like petals and 1D nanopaddy can demonstrate stimulating properties as electrocatalysts for electrochemical applications. These morphologies can improve the contact area between the electrode material and the electrolyte, as well as electrode material and current collector, and they can provide short diffusion pathways for ions/molecules for the active sites, thus enhancing the electrocatalytic activity. The Ni foam with suitable 3D porous structure is used as the current collector and template to grow an active electrode material; because of its high electrical conductivity, it can accelerate the electron and charge transfer processes, and is also advantageous in the easy removal of the generated O2 or H2 gas molecules at the electrode 13 ACS Paragon Plus Environment


surface. Thus, non-precious metal-based electrode materials with constructive architecture on Ni foam can improve the efficiency of electrocatalytic processes. The fabulous morphologies with shapes of natural pattern (flower and paddy leaf) are due to the slow dissociation and diffusion of precipitating anions in the reaction mixture during the hydrolysis of urea, the growth of nanocrystals, and their orientation being kinetically controlled. Metal cations, counter anions, and precipitating agent, temperature, and time are all accountable factors for obtaining the specific morphology of -LDH materials. Figure 3 shows the atomic force microscopy (AFM) images of nanopetal of 3D microflower and nanopaddy leaf of Ni1-xFex-LDH and 1D Co1-xFex-LDH, respectively. AFM is a direct tool that is used to study the thickness of the nanostructured materials. The average thickness of the nanoflake (Figure 3a) and nanopaddy leaf (near the tip) (Figure 3b) are measured to be 8 and 1.5 nm, respectively, as estimated from the height profile. The thickness of nanoleaf is 6 times thinner than the nanopetal. The topographical images of 2D Ni1-xFex-LDH and 1D Co1-xFex-LDH nanoparticles and its measured thickness support the FESEM results (Figure 2).


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Figure 3. (i) 2D AFM images, (ii) height profiles and (iii) 3D AFM images of (a) nanopetal of Ni1-xFex-LDH and (b) nanopaddy leaf of Co1-xFex-LDH. The HRTEM measurements provide better insights into the dimensionality and structural information of the Ni1-xFex-LDH and Co1-xFex-LDH materials. Figure S2a-2c and S2d-2f of the SI show the 1D nanopaddy leaf-like and 2D sheet-like petal shape of the Co1-xFex-LDH and Ni1xFex-LDH

materials, respectively. Figure S2a of the SI shows the bunch of nanoleaves; under the

given synthesis conditions, grain/ripened nanocrystals randomly accumulate in unidirectional manner to form a paddy leaf-like structure with the diameter of ~62 nm (Figure S2c of the SI). The diameter of the nanoleaf decreases during further growth, finally ending up in a sharp tip, usually occurring in the direction of energetically favored structure. Figure S2d of the SI shows the bunch of nanopetals that is separated from the microflower under strong ultrasonication conditions, Figure S2e and S2f of the SI show the horizontally and vertically oriented nanopetals, respectively. Figure 4a and 4b show the high-magnification HRTEM images of Co1-xFex-LDH. 15 ACS Paragon Plus Environment


Figure 4a shows the presence of amorphous shell and crystalline core on the nanopaddy leaf, indicating the polycrystalline nature of the sample; the crystalline core decreases with the growth of the material. The nanoleaf of Co1-xFex-LDH display an interlayer separation of 0.78 nm, corresponding to the (003) basal plane of -LDH crystal which is shown in Figure 4b. The regular layer-by-layer arrangement of brucite layers with interlayer spacing of 0.78 nm is clearly shown in the HRTEM images Figure 4b and 4c (enlarged portion of Figure 4b), where the thickness of the brucite layer was found to be 0.48 nm.52 This indicates that nanoleaves of Co1-xFex-LDH possess a highly (00l) preferred orientation. The selected area electron diffraction (SAED) pattern of Co1-xFex-LDH (Figure 4d) demonstrates crystalline nature of the core part of the nanopaddy leaf. The elemental composition of nanoleaves of Co1-xFex-LDH was analyzed by high angle annular dark field- scanning transmission electron microscopy (HAADF-STEM). Figure 4e shows the presence of Co, Fe, and O elements on the nanoleaves. Further, energy


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Figure 4. (a-c) High magnification HRTEM images of Co1-xFex-LDH, (d) SAED pattern, (e) HAADF-STEM image and corresponding elemental color mapping showing the presence of Co, O and Fe, (f) EDX spectrum, (g-i) HRTEM images of Ni1-xFex-LDH (j) SAED pattern (k) HAADF-STEM image and corresponding elemental color mapping showing the presence of Ni, O and Fe (l) EDX spectrum. dispersive X-ray (EDX) spectroscopy analysis provided further evidence of the existence of Co and Fe elements with an atomic ratio of 1:0.07 (Figure 4f). The Fe doped 1D nanoleaves are 17 ACS Paragon Plus Environment


expected to have higher electronic conductivity. Figure 4g-4i show the high magnification HRTEM images of microflower Ni1-xFex-LDH, which validate the upward facing petals (Figure 4g) and laterally oriented petals (Figure 4h). The hazy lattice fringes and randomly oriented crystals with different facets (dotted circles) in Figure 4h and 4i indicate the polycrystallanity of the sample. The interplanar spacings of Ni1-xFex-LDH were found to be 0.25 nm, which is identical to the (012) plane (inset of Figure 4i). The ring pattern of SAED image (Figure 4j) is also confirms the polycrystalline nature of the sample. The STEM images of Ni1-xFex-LDH nanopetals show the existence and uniform distribution of Ni, Fe, and O elements (Figure 4k). Associated EDX spectroscopy analysis provides further evidence of the presence of Ni and Fe elements with an atomic ratio of 1:0.3 (Figure 4l). EDX profiles of both the materials (Figure 4f and 4l) show the presence of F- ions, indicating the co-existence of some of the fluoride ions in the solution incorporated into the interlamellar spaces of the -LDH. The structural information obtained from the HRTEM analysis is consistent with the PXRD results. Inorder to quantify the Fe content in the catalysts ICP measurement was performed and given in the Table S1 of the SI. The doping of Fe into the Ni and Co-LDH in the atomic scale can increase active centers, enhance the conductivity, and improve the charge efficiency and utilization of active material. Further, in order to get better insights into the surface elemental composition and valence states of Ni1-xFex-LDH and Co1-xFex-LDH, X-ray photoelectron spectroscopy (XPS) has been executed. Survey spectra of Ni1-xFex-LDH and Co1-xFex-LDH (Figure S3a and S3b of the SI, respectively) show the presence of Ni, Fe, O, and C elements and Co, Fe, O, and C elements, respectively; these results further support the EDX obtained from the HRTEM analysis. The deconvoluted spectrum of Ni 2p (Ni1-xFex-LDH) in Figure 5a shows two spin−orbit doublets and two shakeup satellites, which are characteristic of Ni2+ and Ni3+.26,53 The observed two main 18 ACS Paragon Plus Environment

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Figure. 5. XPS analysis: (a) Ni 2p, (b) Fe 2p and (c) O 1s spectra of Ni1-xFex-LDH material; (d) Co 2p (e) Fe 2p and (f) O 1s spectra of Co1-xFex-LDH material. peaks at binding energy values (BE) of 856.50 and 874.20 eV with a spin-orbit splitting of ∼17 eV are ascribed to Ni 2p3/2 (Ni3+ and Ni2+ at 856.2 and 858.2 eV) and Ni 2p1/2 (Ni3+ and Ni2+ at 874.1 and 876.1 eV), respectively. The Ni 2p peaks of Fe-doped Ni-LDH were shifted towards positive energies compared to the un-doped Ni-LDH, which indicates that when Fe is added, the oxidation of Ni2+ is favored.54 The deconvoluted Fe 2p spectra of Ni1-xFex-LDH and Co1-xFex-LDH (Figure 5b and 5e) show two peaks at BE values of 713.5 (Fe 2p3/2) and 725.5 eV (Fe 2p1/2), which signify that iron exists in +3 oxidation state as Fe3+.37,53 Figure 5c and 5f show the O1s deconvoulated spectra of Ni1-xFex-LDH and Co1-xFex-LDH, respectively, while the peaks sited at BE values of 531.0 and 533.5 eV correspond to the characteristic peaks of −OH and physisorbed or chemisorbed water on the surface of sample, respectively.53 The Co2p deconvoluted spectra (Figure 5d) of Co1-xFex-LDH shows two main peaks at BE values of



781.2 and 796.50 eV corresponding to Co 2p3/2 and Co 2p1/2, respectively and two shakeup satellites at 787.0 and 803.0 eV that are characteristic of Co2+ and Co3+, respectively.35,41,53 Thus, XPS data shows that the brucite layers of Ni1-xFex-LDH, and Co1-xFex-LDH contain 2+ and 3+ metal cations; acquiring more positive charge is anticipated to exhibit higher catalytic activity. 2.2. Electrochemical characterization The amorphous electrode materials are known to show higher OER and HER activity compared to the crystalline materials.15,55 Hence, the porous, unique nanopaddy and microflower morphologies of Ni1-xFex-LDH and Co1-xFex-LDH electrode materials are expected to show higher catalytic activity towards OER and HER.15,55,56 In order to study the bifunctional properties, we have taken two individuals of the same electrodes. Initially, OER was studied on one electrode, and HER on the other electrode. After that, the same electrodes were used for the study of overall water splitting. 2.2.1. Electrocatalytic OER study OER at anode is the bottleneck of overall water splitting. In this regard, the OER performance of Ni1-xFex-LDH and Co1-xFex-LDH electrodes was first evaluated in a typical three-electrode electrochemical cell using aqueous 1.0 M KOH solution. Before electrochemical analysis, the electrodes were activated by continuous cyclic voltammetry (CV) cycling for 100 cycles at a scan rate of 20 mV s-1. Figure 6a shows the iR-compensated OER polarization curves of Ni1xFex-LDH

and Co1-xFex-LDH electrodes at a scan rate of 2 mV s−1 on the reversible hydrogen

electrode (RHE) scale. Strikingly, as-prepared Co1-xFex-LDH electrode exhibits overpotential of 165 mV at 10 mA cm-2 and 260 mV at 50 mA cm-2 while the as-prepared Ni1-xFex-LDH at the


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same current densities exhibits 235 mV and 300 mV, respectively which is shown in the inset of Figure 6a. The OER performance of Ni1-xFex-LDH and Co1-xFex-LDH electrodes is remarkably influenced by the incorporated element. Their electrocatalytic activity towards OER is not monotonous, and the Co1-xFex-LDH electrode shows the best activity. The overpotentials of these electrodes are significantly lower than the recently reported benchmark IrO2 and RuO2 catalysts (Figure 6b and Table S2 of the SI).36,57,58 In order to know the material activity on Ni foam, the polarization curve of bare Ni foam was also recorded, and is shown in Figure 6a, which shows much higher overpotentials (358 mV at 10 mA cm-2 and 446 mV at 50 mA cm-2) compared to the Ni foam with deposited materials. This indicates that the shifted overpotential is solely from the deposited electrode material. Figure 6b compares the OER overpotentials of Ni1-xFex-LDH and Co1-xFex-LDH electrodes with recent reports, while Table S2 of the SI gives the large set of comparison. The Ni1-xFex-LDH and Co1-xFex-LDH electrodes show significantly lower overpotentials and higher OER activity compared to the other hydroxide, oxide, oxyhydroxide, sulphide, phosphide, and nitride materials; this indicates that the unique morphologies of Ni1xFex-LDH

and Co1-xFex-LDH electrodes serve as efficient anode catalysts for alkaline water

electrolysis. Noticeably, the overpotentials obtained for Ni1-xFex-LDH and Co1-xFex-LDH are lower than the other stoichiometric ratios of Fe and Ni or Co recently reported (Table S2 of the SI).35,36,53,54,56 It is known that iron plays an important role in the Ni- and Co-based materials for the improvement of OER activity; however, reports are very limited, and the efficiency need to be improved for commercial use.31,59 In the LSV curves, considerable oxidation peaks corresponding to the Ni(II)Ni(III) and Co(II) Co(III) Co (IV) transitions in the potential region of 1.3 to 1.6 V vs. RHE are not observed, due to the strong electronic interactions between the Fe and Ni or Co, which alter the electronic structures of the electrocatalysts, thus making Ni 21 ACS Paragon Plus Environment


Figure 6. OER analysis: (a) LSV curves at a scan rate of 2 mV s-1 (iR-compensated), (b) Overpotentials comparison with recent reports, (c) Tafel curves, (d) multi-current step profiles (iR-uncompensated), (e) CP profiles (f) EIS Nyquist profiles at a potential of 1.5 V (vs. RHE) of Ni1-xFex-LDH and Co1-xFex-LDH electrodes recorded before and after stability test in aqueous 1.0 M KOH. 22 ACS Paragon Plus Environment

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and Co oxidation difficult, while Fe remains as Fe(III) (Fe does not show any redox peaks in this potential region).31,60,61 The doped metal cations can enhance the electron conductivity and facilitate the OH adsorption on the catalyst surface, due to the increased positive charge cloud on the mixed metal ions, which is higher in the case of Co1-xFex-LDH compared to Ni1-xFexLDH.31 The lowest overpotential for Co1-xFex-LDH electrode indicates that this material needs very low OER activation energy, and the rate reaction kinetics is higher. In order to demonstrate the role of Fe-doping, the individual Ni(OH)x Co(OH)x, and Fe(OH)x, samples are tested by LSV and compared with the doped ones, which is shown in Figure S4 of the SI. The Figure S4 of the SI shows that the Fe-doped materials (Ni1-xFex-LDH, and Co1-xFex-LDH) exhibit higher catalytic performance compared to the undoped ones (Ni(OH)x, and Co(OH)x) as well as RuO2 electrode. It is also known that the electrochemical performance strongly depends on the morphology and porosity, the homogeneously grown porous nanopaddy-like morphology of Co1-xFex-LDH providing abundant electroactive sites to be exposed to OH ions. Usually during OER, the attachment of generated O2 gas molecules on electrode surface blocks the active sites, and diminishes further oxidation, which is one of the reasons to increase overpotential.55 Especially, this is an obvious issue for high efficient electrodes; under high current operating condition, more gas bubbles will be generated, due to the faster reaction kinetics. The porous nanopaddy morphology of Co1-xFex-LDH is found to be very effective in dispelling the gas bubbles, the reason for the high OER activity. The TEM observations signify that the exposed layered-type structure (Figure 4c) is more favourable for high OER catalytic activity. In order to obtain further insights into these electrodes, Tafel plots were derived from the corresponding LSV curves 23 ACS Paragon Plus Environment


according to the Tafel equation η = blog(j/j0), shown in Figure 6c, where η is the overpotential, j is the current density, j0 is the exchange current density, and b is the Tafel slope. The estimated Tafel slopes for Ni1-xFex-LDH and Co1-xFex-LDH materials are 72 and 36 mV dec-1, respectively. The smaller slope indicates the higher reaction kinetics for oxygen evolution.31 The electrochemical active surface area (ECSA) of these electrode materials was estimated using an established method ECSA = Cdl/Cs equation, where Cdl is the electrochemical double-layer capacitance, and Cs is the specific capacitance of a flat smooth surface of the electrode material, which according to literature reports is assumed to be 40 mF cm-2.62 Cyclic voltammograms of Ni1-xFex-LDH and Co1-xFex-LDH electrodes are recorded in a non-Faradaic region (1.02–1.12 V vs. RHE) at scan rates of 40, 50, 60, 70, 80, 90 and 100 mV s-1 (Figure S5a and S5b of the SI, respectively), and then Cdl was evaluated by plotting the difference of the anodic and cathodic current densities (Δ j = ja-jc) at 1.1 V vs. scan rate (Figure S5c and S5d of the SI). The obtained linear slope is twice that of the Cdl, Co1-xFex-LDH exhibiting a larger Cdl (5.28 mF cm-2) compared to Ni1-xFex-LDH (1.76 mF cm-2), indicating that Co1-xFex-LDH has exposed a greater number of electrocatalytic active sites. Figure 6d shows the multi-current steps curves of Ni1-xFex-LDH and Co1-xFex-LDH electrodes recorded at current densities varying from 10 to 200 mA cm-2. With increasing current density, the potential of both electrodes rises accordingly, and stabilizes rapidly at all regions of current densities. These responses reflect the excellent mass transfer property (inward diffusion of OH and outwards diffusion of O2 bubbles) and mechanical stability of Ni1-xFex-LDH and Co1-xFex-LDH electrodes.55 Note that the resultant potentals at respective current densities in Figure 6d are higher than the potential values of LSV curves (Figure 6a) due to the iRuncompensation of multi-current steps curves. However, for Co1-xFex-LDH electrode, the 24 ACS Paragon Plus Environment

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overpotentials at all current densities are much lower than those of the Ni1-xFex-LDH electrode (Figure 6d), which follows the same trend observed in LSV. For practical applications of the water-splitting systems, stability is an important consideration. To evaluate the durability in an alkaline milieu, continuous potential cycling was performed at a constant current density (J) of 50 mA cm-2. Figure 6e shows the chronopotentiometry profile of both electrode materials; in both cases, the potential is maintained almost the same, even after one and a half days (36 h). For the initial few hours, a slight increase in the potential was seen, which is due to the peeling of loosely bound electrode particles at the outer surface during the rapid generation and liberation of gas molecules at high current condition. After that, the detachment of electrode material from the Ni foam is not observed, indicating excellent robustness. We recorded the polarization and multi-current steps curves in a freshly prepared aqueous 1.0 M KOH after 36 h, which are shown in Figure 6a and 6d, respectively; we find that there is a decrease in the potential in the case of Ni1-xFex-LDH, while no considerable change is shown in Co1-xFex-LDH; however, Co1-xFexLDH electrode shows superior OER activity, even after the CP stability test. Further, in order to assess the long-term durability, we have carried out 1000 CV cycles on Co1-xFex-LDH electrode at a scan rate of 50 mV s-1, as shown in Figure S6 of the SI, which is a very important test for evaluating the stability of an electrode catalyst, but which has not been carried in many recent reports.16,18,24–26 Interestingly, the overpotential tends to decrease with the increase of cycles, which is a good sign of favorable anode electrocatalyst for OER (inset of Figure S6 of the SI). After CV stability test, the OER performance is compared with the initial one by recording the LSV and multi-current steps curves in a freshly prepared aqueous 1.0 M KOH, shown in Figure S7a and S7b of the SI. Interestingly, the overpotentials have decreased after the CV stability test (Figure S7a and S7b of the SI), indicating improved OER catalytic activity. Thus, Co1-xFex-LDH 25 ACS Paragon Plus Environment


electrode only demands a potential of 1.52 V to carry a high current density of 200 mA cm-2, while Ni1-xFex-LDH needs 1.65 V at same current density (Figure 6a and S7a). In both the materials, the improved activity with the stability tests could be due to morphological/structural, compositional, and electronic structural changes. The multi-current steps results (Figure 6d) are well matched with the LSV results (Figure 6a). The time-dependent potential curve (Figure 6e) and continuous CV cycles (Figure S6 of the SI) validate the excellent durability of Ni1-xFex-LDH and Co1-xFex-LDH electrodes for OER. After CV stability test, a clear oxidation peak is observed in the LSV curve of Co1-xFex-LDH in the potential region of 1.45-1.5 V; this is ascribed to the oxidation of cobalt ion Co (II/III/IV), which indicates that during repeated CV cycles the material is undergoing partial oxidation. The increased number of Co(IV) sites can enhance the OER activity by means of reducing the overpotential and increasing the current density (Figure S6 of the SI).22 Although in the beginning Ni1-xFex-LDH has higher overpotential, after CP stability test the values are decreased (Figure 6a), while Co1-xFex-LDH shows extraordinary catalytic activity and stability. The very high OER performance of the Co1-xFex-LDH electrode can be ascribed to several factors: (i) in situ, binder-free, and uniformly grown nanopaddy-like catalyst on Ni foam exhibits high electrical conductivity, and diminish the resistance rising from the contact between Ni foam and catalysts; (ii) the unique hierarchically structured porous nanopaddy-like morphology of Co1-xFex-LDH on Ni foam configuration improves wettability, provides large electroactive surface area and enhances rapid gas bubble dissipation ability; and (iii) low solution resistance (Rs, 1.05 ) and charge transfer resistance (Rct, 0.83 ) compared to Ni1-xFex-LDH (Rs=1.22  and Rct=0.99 ), which is shown in Nyquist plot (Figure 6f and S8a of the SI); these values are decreased after stability tests, which are shown in Figure S8a and S8b, respectively, of the SI, indicating the improved charge transfer kinetics. 26 ACS Paragon Plus Environment

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Some of the density functional theory (DFT) calculations in the recent reports show that the free energy change of each reaction step will play a vital role for the OER activity of electrocatalysts. The proposed OER mechanism on electrode surface in alkaline electrolyte is shown below.63–65 𝑀 + 𝑂𝐻 ― →𝑀 ― 𝑂𝐻 + 𝑒 ―

∆𝐺 > 0 (5)

𝑀 ― 𝑂𝐻 + 𝑂𝐻 ― →𝑀 ― 𝑂 + 𝑒 ― + 𝐻2𝑂 𝑀 ― 𝑂 + 𝑂𝐻 ― →𝑀 ― 𝑂𝑂𝐻 + 𝑒 ―

∆𝐺 < 0 (6) ∆𝐺 > 0 (7)

𝑀 ― 𝑂𝑂𝐻 + 𝑂𝐻 ― →𝑀 ― 𝑂2 + 𝑒 ― + 𝐻2𝑂 ∆𝐺 < 0 (8) 𝑀 ― 𝑂2 →𝑀 + 𝑂2↑

∆𝐺 < 0

(9)

where, M refers to the metal cation. Various intermediates, such as M–OH, M–O, M–OOH, and M–O2 are formed, and finally release the O2 gas molecules. The optimal free energy changes obtained by DFT calculations corresponding to the intermediates and products formed during OER are shown in the above mechanism. The intermediate steps (5)-(8) in the above mechanism are the adsorption of *OH, *O, *OOH, and O2 intermediates, respectively, on metal-LDH. The free energy change shows that steps (6), (8), and (9) are spontaneous, while steps (5) and (7) have energy barriers.63–65 Therefore, steps (5) and (7) are found to be rate-limiting steps. More positively charged cation species tend to adsorb more OH ions in solution for charge balance, and lower its free energy change. It is known that metal doped-LDH materials will have lower free energy changes compared to the un-doped ones, as dopants can generate higher valences in the layered structures.65 Between the two Fe doped-LDH materials, Co1-xFex-LDH acquires more positive charge compared to Ni1-xFex-LDH, which is one of the reasons for the higher OER catalytic activity. It is proposed that the generated Co(IV) sites under a given OER potential can 27 ACS Paragon Plus Environment


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increase the electrophilicity of the adsorbed O species (Equation 4), thereby facilitating the formation of O-OH (Equation 4) via incoming nucleophile (OH) attack. The Co(IV) cations can also stimulate the deprotonation of the OOH species by electron-withdrawing inductive effect to form O2 (Equation 5).63 The impressive catalytic activity and remarkable stability (Figure 6b, 6e and Table S2 of the SI) of low cost Co1-xFex-LDH, as well as Ni1-xFex-LDH electrodes, enable promising alternative candidates for OER to replace precious metal-based electrocatalysts for overall water splitting. Thus, these electrodes can be used as a promising anode candidate for large-scale alkaline water electrolysis. 2.2.2. Electrocatalytic HER study The HER electrocatalytic activity of Ni1-xFex-LDH and Co1-xFex-LDH electrodes was studied in aqueous 1.0 M KOH electrolyte using a typical three-electrode setup. Figure 7a shows the iRcompensated LSV curves at a scan rate of 2 mV s-1 on the RHE scale. In the beginning, as expected, Co1-xFex-LDH has shown lower overpotential for HER activity compared to Ni1-xFexLDH. The overpotentials for Ni1-xFex-LDH electrode at 10 and 50 mA cm-2 are 242 and 319 mV, respectively, and for Co1-xFex-LDH electrode are 205 and 273 mV, while for bare Ni foam are 303 and 489 mV at same current densities respectively, which is shown in the inset of Figure 7a. These overpotentials of Ni1-xFex-LDH and Co1-xFex-LDH are lower than the recently reported non-noble-metal HER catalysts (Figure 7b), and Table S3 of the SI gives the detailed comparison.66 The plausible HER mechanism on electrode surface in alkaline medium is shown below.67,68 𝑀 + 𝐻2𝑂 + 𝑒 ― →𝑀 ― 𝐻 ∗ (𝑉𝑜𝑙𝑚𝑒𝑟 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛)

(10)

𝑀 ― 𝐻 ∗ + 𝐻2𝑂 + 𝑒 ― →𝑀 + 𝑂𝐻 ― + 𝐻2 (𝐻𝑒𝑦𝑟𝑜𝑣𝑠𝑘𝑦 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛) (11) (𝑜𝑟) 28 ACS Paragon Plus Environment

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2𝑀 ― 𝐻 ∗ →2𝑀 + 2𝐻2 (𝑇𝑎𝑓𝑒𝑙 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛)

(12)

Figure 7. HER analysis: (a) LSV curves at a scan rate of 2 mV s-1 (iR-compensated), (b) Overpotentials comparison with recent reports, (c) Tafel curves, (d) multi-potential step profiles (e) CP profiles (f) EIS Nyquist profiles at a potential of -0.125 V (vs. RHE) of Ni1-xFex-LDH and Co1-xFex-LDH electrodes recorded before and after stability test in aqueous 1.0 M KOH. 29 ACS Paragon Plus Environment


where, M represents metal cation, and M–H* designates an intermediate hydrogen atom chemically adsorbed on an active metal site. These paths are mostly dependent on the inherent electronic and chemical properties of the electrode surface.67,68 The reaction steps of Equation (10), (11), or (12) are the possible rate-limiting steps, which can be discriminated by estimating the Tafel slope value from the polarization curve.68 According to the Sabatier principle,67 an efficient HER catalyst should have high affinity to form a strong bond with H* to enable the proton-electron-transfer process, while at the same time, it should also be sufficiently weak to a facile bond breaking for a release of gaseous H2. It is found that Fe-doped transition metal LDHs can have the lower gibbs free energy (ΔGH*) compared to the single metal-LDH and hence Fedoping boost the hydrogen evolution activity.69 Figure 7c shows the Tafel plots of Ni foam, Ni1xFex-LDH,

and Co1-xFex-LDH, which are derived from the corresponding LSV curves. The Tafel

slope of Co1-xFex-LDH is 98 mV dec-1, while Ni1-xFex-LDH exhibits 110 mV dec-1. This signifies that the Co1-xFex-LDH electrode material has better kinetics compared to the Ni1-xFex-LDH. Figure 7d shows the multi-potential step curves recorded from -0.8 to -1.5 V (vs. Ag/AgCl) with -0.1 V potential increments per 100 s. At all potentials, the current immediately levels off, and maintains almost constant in the remaining 100 s, signifying the excellent mass transportation, mechanical robustness, and conductivity.62 However, the electrochemical durability of the electrode is very important, apart from good catalytic activity for an efficient HER electrode. In order to study the durability, electrodes were kept at higher cathodic current density of -50 mA cm-2 under rapid hydrogen evolution for one and a half days (36 h) (Figure 7e). Fascinatingly, the overpotential is decreased after 36 h when compared to the initial value, indicating excellent HER catalytic activity. Strikingly, the decrease in potential with time is more for the Ni1-xFexLDH electrode, signifying that the HER activity on the Ni1-xFex-LDH electrode is highly 30 ACS Paragon Plus Environment

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sensitive, compared to that on the Co1-xFex-LDH electrode. After CP stability test, we have recorded the LSV and multi-potential step curves in a freshly prepared aqueous 1.0 M KOH, shown in Figure 7a and 7d, respectively. These show that after the CP test, the HER catalytic activity drastically increased, and Ni1-xFex-LDH electrode shows the best activity. The increased HER activity could be due to change in the morphological orientation, increase in the wettability, and enhancement in the charge transfer kinetics during the rapid evolution of hydrogen gas bubbles. In order to know the charge transfer kinetics, electrochemical impedance spectroscopy has been carried out at a potential of -0.125 V vs. RHE, before and after CP test, Figure 7f shows the Nyquist plots of Ni1-xFex-LDH and Co1-xFex-LDH; before the CP test, Co1-xFex-LDH shows less charge transfer resistance compared to Ni1-xFex-LDH. However, after the CP tests, the charge transfer resistance has been significantly decreased for both the electrode materials (Figure S9 of the SI). Among them, Ni1-xFex-LDH after the CP test has shown the lowest charge transfer resistance, which is the reason for high HER catalytic activity. Thus, the Ni1-xFex-LDH and Co1-xFex-LDH electrodes show much superior HER activity under alkaline medium (pH = 14), as compared to the other non-precious HER catalysts recently reported (Figure 7b, and Table S3 of the SI). The Ni1-xFex-LDH and Co1-xFex-LDH electrodes need only 170 and 183 mV overpotential, respectively, to drive 10 mA cm-2 current density after long-time stability test. The outcomes obtained from LSV before and after the stability tests agree well with the multipotential step and EIS results, confirming the good reliability. 2.2.3. Electrochemical overall water splitting study using Ni1-xFex-LDH and Co1-xFex-LDH bifunctional electrodes According to the OER and HER studies, the Ni1-xFex-LDH and Co1-xFex-LDH materials on Ni foam are found to be very active and stable catalysts in alkaline medium. These electrocatalysts 31 ACS Paragon Plus Environment


Figure 8. (a) Half cell OER and HER LSV curves of Ni1-xFex-LDH and Co1-xFex-LDH electrodes at a scan rate 2 mV s-1 (iR-compensated); Full cell overall water splitting: (b) LSV curves of Co1-xFex-LDH(+)ǁCo1-xFex-LDH(-), Ni1-xFex-LDH(+)ǁNi1-xFex-LDH(-) and Co1-xFexLDH(+)ǁNi1-xFex-LDH(-) electrolyzers at a scan rate 2 mV s-1 (iR-uncompensated), (c) comparison of cell potentials with recent reports, (d) CA profiles of Co1-xFex-LDH(+)ǁCo1-xFexLDH(-) and Ni1-xFex-LDH(+)ǁNi1-xFex-LDH(-) electrolyzers (e) Schematic diagram of alkaline water electrolysis cell with different combinations of anode an cathode catalysts.


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can be used as an anode, as well as a cathode, in an alkaline electrolyzer, as per requirements. For an ideal electrolyzer, the voltage difference between the OER and HER should be very low. Figure 8a show the voltage difference (cell potential, Ecell) between OER and HER of Ni1-xFexLDH and Co1-xFex-LDH catalysts. The cell potential (Ecell=Eanode+Ecathode) at a current density of 10 mA cm-2 for Co1-xFex-LDHǁCo1-xFex-LDH, Ni1-xFex-LDHǁNi1-xFex-LDH, and Ni1-xFexLDHǁCo1-xFex-LDH are found to be 1.58, 1.61 and 1.57 V, respectively. These values are much lower than the values recently reported of NiFe hydroxides (1.64 V).27 From the half-cell electrode measurements, it is observed that both Ni1-xFex-LDH and Co1-xFex-LDH electrodes have shown efficient bifunctional properties for OER and HER. However for practical applications, it is very important to study their real performance in an alkaline electrolyzer. Three types of electrolyzers have been fabricated using Ni1-xFex-LDH and Co1-xFex-LDH electrodes (both are tested electrodes for OER and HER) as anode, as well as cathode, such as Ni1-xFexLDH(+)ǁNi1-xFex-LDH(-), Co1-xFex-LDH(+)ǁCo1-xFex-LDH(-), and Co1-xFex-LDH(+)ǁNi1-xFexLDH(-). LSV, multi-potential step and CA have been performed, to study their overall water splitting performance. Figure 8b shows the LSV profiles (iR-uncompensated) of three different combinations of electrolyzers, remarkably, all these combinations have shown very good overall water splitting performance. The continuous evolution of O2 at anode and H2 at cathode for three combinations of electrolyzers are shown in Video S1, S2, and S3 of the SI. The rapid generation and liberation of O2 and H2 molecules at lower potentials signifies the high catalytic and bifunctional activities of the electrodes. The cell potentials of the Co1-xFex-LDH(+)ǁCo1-xFexLDH(-), Ni1-xFex-LDH(+)ǁNi1-xFex-LDH(-), and Co1-xFex-LDH(+)ǁNi1-xFex-LDH(-) electrolyzers at current densities of 10 and 50 mA cm-2 are 1.6 , 1.6 and 1.59 V, and 1.76 , 1.77 and 1.76 V, respectively which are shown in the inset of Figure 8b. These values are lower than the recently 33 ACS Paragon Plus Environment


reported materials shown in Figure 8c.70 Table S4 of the SI provides the comparison with the large set of recently reported electrode materials. The Ecell values obtained in the two-electrode system at current density of 10 mA cm-2 are very close to the values obtained in three-electrode systems (Figure 8a and 8b), which indicates the excellent reliability of the results. Figure S10 of the SI shows the multi-potential step profiles of three electrolyzers recorded from 1 to 2 V with 0.2 V potential increments per 100 s. The current is gradually increased and stabilized with the increase of potential, which signifies the excellent mass transfer property, as well as mechanical durability. Further, the stabilities of the Ni1-xFex-LDH(+)ǁNi1-xFex-LDH(-), and Co1-xFexLDH(+)ǁCo1-xFex-LDH(-) electrolyzers have been tested at 1.7 V for 1 day (24 h), which is shown in Figure 8d. These electrolyzers maintained stable current during continuous evolution of O2 at H2 gas bubbles at anode and cathode, respectively, indicating their excellent robustness for overall water splitting. Thus the Ni1-xFex-LDH and Co1-xFex-LDH electrodes exhibit outstanding stability during the entire session tested herein (OER, HER, and overall water splitting) (Figure 6e, 7e and 8d). The remarkable electrocatalytic activity, as well as stability, of both these electrodes can be ascribed to the following factors: i)-LDHs (brucite layers) with mixed valences of M2+ and M3+ (M=Ni/Co/Fe) likely have intrinsically high and bi-functional catalytic activities for OER and HER. The existence of high valence cations in -LDHs can easily form MOOH during OER, thereby facile generation of O2 via deprotonation. ii) The low level of Fe doping inLDH enhances the conductivity and electrochemical active centers. The Ni, Co, and Fe cations are found to be electroactive centers with unique electronic structures in a definite crystalline structure, thus promoting higher catalytic activity. iii) The unique chrysanthemum microflower and nanopaddy morphologies grown on 3D porous Ni foam provide a large number of exposed active sites, reduce the diffusion path length, and enhance the process of ion diffusion kinetics 34 ACS Paragon Plus Environment

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and facilitate liberation of generated gas bubbles. v) The binder-free in situ growth of active material on Ni foam enhances intimate contact between the current collector and electrode material, ensuring efficient charge transfer. 2.2.4. Post physical characterizations The electrochemical results show that the OER, HER, and overall water splitting performance have been increased with the stability tests. The electrode materials may undergo considerable changes under the high applied potentials of OER and HER, due to the vigorous gas evolution. Reliable studies in the literature on electrocatalysts during or after the OER and HER are often impaired. Herein, we have employed the optical, PXRD, FESEM, HRTEM and XPS techniques, in order to find the changes in the electrodes after all electrochemical studies. Figure S11a(i–iii) and S11b (i–iii) of the SI show the optical images of Ni foams with the deposited Co1-xFex-LDH and Ni1-xFex-LDH electrode materials, respectively, taken before and after OER and HER measurements. In both cases after OER, the color of the materials turned to black (Figure S11a(ii) and S11b(ii) of the SI, respectively), which is more in the Co1-xFex-LDH case, which could be due to the partial oxidation of electrode materials and Ni foam. After HER, the color of the material was not considerably changed (Figure S11a(iii) and S11b(iii) of the SI, respectively). Figure S12a and S12b of the SI show the PXRD profiles of Ni1-xFex-LDH and Co1-xFex-LDH, respectively, before and after OER and HER. After OER and HER, the two electrodes show the same PXRD pattern other than the peak intensities; the enhanced peak intensities indicate that both the materials underwent crystallization during stability tests under the rapid evolution of O2 and H2. The partially oxidized phase of electrode and Ni foam peaks after OER were not observed in PXRD, which indicates minute oxidation. However, the diffraction peaks shifted towards higher angle after OER, which was not observed after HER (insets of Figure S12a and 35 ACS Paragon Plus Environment


S12b of the SI). These results are consistent with the color change in the electrode materials after OER and HER (Figure S11a and S11b of the SI, respectively). Figure S13 and S14 of the SI show the FESEM images of Ni1-xFex-LDH and Co1-xFex-LDH, respectively. After the long-term CP and CV stability tests (for OER) the microflower morphology of Ni1-xFex-LDH underwent slight deformation (Figure S13a-13c of the SI), due to the rapid evolution of O2 gas bubbles; however this has shown a positive effect on the catalytic activity, even after stability tests. While the morphology was undisturbed under HER stability test (Figure S13d-13f of the SI). Interestingly, after OER as well as HER stability tests, some of the nanopaddy leaves of the Co1xFex-LDH

ends came into closer contact, and formed a unique structure with large porosity

(Figure S14a-14f of the SI). The ultrathin (

Remarkable Bifunctional Oxygen and Hydrogen Evolution

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