Optimizing NiâFe Oxide Electrocatalysts for Oxygen Evolution

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Optimizing Ni-Fe Oxide Electrocatalysts for Oxygen Evolution Reaction by Using Hard Templating as a Toolbox Mingquan Yu, Gun-hee Moon, Eckhard Bill, and Harun Tüysüz ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01769 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Energy Materials

Optimizing Ni-Fe Oxide Electrocatalysts for Oxygen Evolution Reaction by Using Hard Templating as a Toolbox Mingquan Yua, Gunhee Moona, Eckhard Billb and Harun Tüysüz*a a Max-Planck-Institut

für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr,

Germany. b

Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim an der

Ruhr, Germany.

Keywords: oxygen evolution reaction, nickel iron oxides, hard templating, metal stoichiometry, activation, practical electrode

Abstract A specific investigation was carried out to study the influence of Ni/Fe ratio for oxygen evolution reaction (OER) by using hard templating method as a toolbox. Various compositions of homogeneously blended Ni-Fe oxide nanoparticles with a primary particle size of around 8 nm were simply prepared by using pore confinement of the tea leaves template. Based on the similar physical properties including particle size and surface area for all samples, it was verified that the OER activity in alkali electrolyte was mainly governed by the metal stoichiometry, where a maximum current density was obtained with Ni/Fe ratio of 32/1. The higher catalytic performance of Ni32Fe was attributed to lower reaction resistance and higher intrinsic activity, which are confirmed by electrochemical impedance spectroscopy and surface area analysis,

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respectively. The lowest overpotential (0.291 VRHE at 10 mA/cm2) as well as the highest current density (over 600 mA/cm2 at 1.7 VRHE) was achieved with Ni/Fe 32/1 loaded on nickel foam due to (i) an uniform distribution of Fe into NiO, (ii) a high conductivity, and (iii) an activation of Ni by neighboring Fe under applying bias. The environmentally-benign surfactant-free synthetic procedure and the electrocatalytic system consisting of earth-abundant elements only (Fe, Ni, and O) should be attractive for the development of practical and economical energy conversion devices to split water.

Introduction Electrochemical water splitting is a clean and promising technology for the storage of renewable energy in the form of hydrogen fuel gas.1,2 Due to the multi-electron transfer with sluggish kinetics, the anodic oxygen evolution reaction (OER) has been considered as the bottleneck in the realization of efficient water splitting.3,4 Much larger overpotentials are required to drive this anodic reaction in comparison to the hydrogen evolution reaction (HER) conducted on the cathodic side. Therefore, the development of efficient OER electrocatalysts is highly desired to promote the overall reaction kinetics of water splitting process. Although noble-metal oxide electrocatalysts, such as IrO2 and RuO2, have exhibited good OER activity, still regarded as the state-of-art OER catalysts,2,5,6 their large-scale application is not favorable in terms of the scarcity, high cost, and unsatisfied stability at high anodic potential as well. In this regard, there is a need to design cost-effective and robust OER catalysts without sacrificing catalytic performance. Among various promising alternatives to noble-metal catalysts, transition metal oxides (e.g., Co3O4, Fe2O3, Mn3O4, NiO, etc.) have attracted growing interest owing to their earth-


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abundance and intrinsic stability in alkaline solution.7-13 In particular, Ni-Fe oxide has been intensively studied since it is well-known as one of the most active OER electrocatalysts for alkaline water electrolysis.14-20 Whereas it is acknowledged that Fe plays a critical role in enhancing the intrinsic activity, the influence of Fe on the improved OER kinetics is still under debate. Regarding this, many different mechanistic hypotheses have been proposed. In principle, the Fe incorporation can increase the conductivity of metal (Co and Ni) oxides which serve as a scaffold for Fe active sites.21-23 Very recently, the in-situ formation of Fe4+ in Ni-Fe oxide during OER process was proved by surface-interrogation scanning electrochemical microscopy and Mössbauer spectroscopy.24,25 Accordingly, there is no doubt that the synergistic effect between Ni and Fe contributes to superior OER activity over pure nickel or iron oxide.14-17 The catalytic performance of Ni-Fe oxide strongly depends on the specific metal stoichiometry,19,26 so it is important to optimize the ratio of Ni to Fe as well as to identify the effect towards different catalytic parameters for OER. Massive efforts have been devoted to targeting the optimal Ni/Fe ratio, where the optimized OER activity was confirmed on Ni-Fe oxide electrocatalysts containing 10-50 mol % Fe.16,27-29 Landon et al. systematically studied NiFe oxide materials synthesized via three different approaches: evaporation induced selfassembly, silica hard templating, and dip coating, where the OER activity was maximized by the incorporation of Fe around 10 mol %.15 Louie et al. carried out a detailed investigation on Ni-Fe oxide thin film fabricated by electrodeposition, and the highest activity, roughly 2 and 3 orders of magnitude higher than Ni and Fe films, respectively, was confirmed when 40 mol % Fe was composed.16 Fominykh et al. reported a solvothermal method to prepare ultra-small Ni-Fe oxide nanocrystals, which demonstrated that Fe0.1Ni0.9O had the highest OER activity, even outperforming IrO2 electrocatalyst.20 In addition to metal composition, physical properties such


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as morphology and specific surface area have significant effect on OER performance since more active sites can be exposed on the surface together with facile accessibility of electrolyte. Nevertheless, these factors were not well controlled in these reports which optimized the Ni/Fe ratio by comparing the OER activity. Therefore, it is of importance to study the effect of the metal composition on OER in a Ni-Fe oxide system with controlled morphology and surface areas considering they are major catalytic parameters for the heterogeneous OER. In this work, we have used the hard templating method as a toolbox to prepare a range of the Ni-Fe oxide compositions with same textural parameters, morphology, and particle size to precisely study effect of the composition of electrocatalyst for OER. Hard templating (nanocasting) is one of the well-known methodologies to fabricate well-defined nanostructure by duplicating the mother morphology as a result of its strict pore confinement.30-32 Mesoporous silica have been widely utilized as templates to synthesize structured and nanosized metal oxides.31,33,34 We have been using silica templated ordered mesoporous metal oxides as model system to explore the key physical and chemical parameters and develop more effective electrocatalysts for OER.35-37 Since the synthesis and removal of silica templates always require an energy-intensive process, some alternative sustainable templating approaches should be developed. In this regard, the direct utilization of carbon based wastes (e.g., food processing wastes, sewage, polymers, textiles, etc.) as templates should be one promising way to produce large scale nanostructured materials.38 This work utilizes spent tea leaves (STL) as a sustainable hard template which is an extension of our previous work for fabrication of nanostructured metal oxides.39 By taking the advantage of pore confinement of tea leaves, uniform structured metal oxide nanocrystals with controlled physical properties can be obtained after impregnation, drying and template removal steps. After a detailed and systematic characterization, the materials were


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employed as OER electrocatalysts in Fe impurities-free KOH electrolyte where a clear composition dependent trend was observed. All the mixed oxides samples indicated a much higher performance than the pristine oxides and the highest activity was achieved with Ni/Fe ratio of 32. Furthermore, these electrocatalysts go through an interesting activation process. Electrochemical impedance spectroscopy and surface area analysis were conducted on STLtemplated oxides, showing lower resistance, larger exposed ECSA and superior intrinsic activity for mixed oxides, which could contribute to their higher OER activity over monometal oxide. For large scale practical applications, the Ni-Fe oxide with optimal ratio was further loaded on a conductive metal substrate (Ni foam) and utilized as anode for electrochemical water splitting. This electrode delivered a high current density of 500 mA/cm2 at 1.677 VRHE and showed good stability during long-term water electrolysis. The highly active electrode from a facile and scalable preparation method could be economically attractive for the large-scale water electrolysis.

Experimental Section All the chemicals and reagents except tea leaves were purchased from Sigma-Aldrich and used as received. Nickel foam was obtained from Racemat BV. Ni foam was cleaned with 3.5 M HCl solution in an ultrasound bath for 10 min and then rinsed with 18.2 MΩ·cm H2O. Synthesis of spent tea leaf templated NiO, Fe2O3 and mixed oxides. The tea leaves (Goran Mevlana) were washed with boiled water for several times until there was no visible colour of washing water, and could be employed as template after drying in air at 90 oC. In a typical hard-templating process, 4 mmol of metal precursor was dissolved in distilled water (20 mL) with pre-washed tea leaf. The mixture was then conducted at room temperature for 2 h.


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Nickel nitrate hexahydrate and iron nitrate nonahydrate were used as precursors and mixed with the designed molar ratio while keeping the total molar of metal precursors at 4 mmol. The mass ratio of pre-washed tea leaf and metal precursors was controlled at 2:1 throughout the experiment. After drying in air at 90 oC overnight, the obtained mixture was calcined at 550 oC for 4 h with a ramping rate of 2 oC min-1. Followed by treating in 0.1 M HCl solution (40 mL) to remove some impurities that are coming from the tea leaves, the final product was collected by centrifugation and repeated washed with distilled water (40 mL portions). Synthesis of non-templated nickel iron oxide. Following the above procedure, nontemplated nickel iron oxides were synthesized by mixture of iron and nickel precursors, and directly collected after calcination where treatment in diluted HCl solution was not conducted. Purification of KOH electrolyte. For rigorously Fe-free electrochemical measurements, the KOH electrolyte was purified according to the procedure reported by Boettcher’s group.23 In brief, 99.999% Ni(NO3)2·6H2O (2 g) was dissolved in 18.2 MΩ·cm H2O (4 mL) in a H2SO4cleaned polypropylene (PP) centrifuge tube. 1 M KOH (20 mL) was added into the tube in order to precipitate high-purity Ni(OH)2. Then, the mixture was shaken for 30 min and the supernatant was decanted after centrifugation. The green solid was then washed three times by adding 18.2 MΩ·cm H2O (20 mL) and 1 M KOH (2 mL) with redispersing the solid, centrifuging, and decanting the supernatant. The obtained solid was ready for purification. 1 M KOH (40 mL) was added in the tube to re-disperse the solid with mechanical shaking for 30 min, followed by 3 h of resting. The mixture was then centrifuged and the KOH supernatant was transferred into a H2SO4-cleaned PP bottle for another round of centrifugation. Finally, the purified KOH supernatant was decanted and stored in a H2SO4-cleaned PP bottle.


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Materials characterizations. Thermogravimetry (TG) measurements were performed on a Netzsch STA 449C thermal analyzer. The thermal treatment was conducted with a heating rate of 5 °C/min up to 800 °C in a mixture gas of argon and oxygen. Transmission electron microscopy (TEM) images of Ni-Fe oxides were obtained with an H-7100 electron microscope (100 kV) from Hitachi. High resolution TEM (HR-TEM) and scanning electron microscopy (SEM) images were taken on HF-2000 and Hitachi S-5500 microscopes, respectively. Energy dispersive X-ray spectroscopy (EDX) was conducted using a Hitachi S-3500N. The microscope is equipped with a Si(Li) Pentafet Plus-Detector from Texas Instruments. N2-physisorption isotherms were measured using 3Flex Micrometrics at 77 K. Prior to the measurements, the samples were degassed at 150 °C for 10 h. Brunauer-Emmett-Teller (BET) surface areas were determined from the relative pressure range between 0.06 and 0.2. The total pore volume was calculated by utilizing the adsorbed volume at a relative pressure of 0.97. Wide-angle X-ray diffraction (XRD) patterns collected at room temperature were recorded on a Stoe theta/theta diffractometer in Bragg-Brentano geometry (Cu K Kα1/2 radiation). X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Kratos HSi spectrometer with a hemispherical analyzer. The monochromatized Al Kα X-ray source (E = 1486.6 eV) was operated at 15 kV and 15 mA. An analyzer pass energy of 40 eV was applied for the narrow scans. Hybrid mode was used as lens mode. The base pressure during the experiment in the analysis chamber was controlled at 4×10-7 Pa. The binding energy scale was corrected for surface charging by use of the C 1s peak of contaminant carbon as reference at 284.5 eV. For Mössbauer measurements, the radioactive source for the 57

27Co

57Fe

Mössbauer transition consists on

isotope diffused into an Rh matrix using a spectrometer with liquid helium flow cryostat

(Oxford Instruments VARIOX). All data were collected at room temperature or 4.2 K and in the


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absence of an applied magnetic field. Isomer shifts and quadrupole splitting experimental values were determined by fitting spectra with Lorentzian lines through the use of the software mf which is developed by Dr. Eckhard Bill. Electrochemical measurements. Electrochemical water oxidation measurements were carried out in a typical three-electrode configuration using a rotating disc electrode (Model: AFMSRCE, PINE Research Instrumentation), a hydrogen reference electrode (HydroFlex, Gaskatel) and Pt wire were used as reference electrode and counter electrode respectively. All electrochemical cell components were cleaned with H2SO4 (1 M) and rinsed with 18.2 MΩ·cm H2O several times. Purified KOH solution (1 M) was used as the electrolyte and argon was purged through the cell to remove oxygen for 30 min to remove oxygen before test. The temperature of the cell was kept at 25 oC by using a water circulation system. Working electrodes were fabricated by depositing the prepared materials on glassy carbon (GC) electrodes (PINE, 5 mm diameter, 0.196 cm2 area). Before use, the surface of the GC electrodes was polished with Al2O3 suspension (5 and 0.25 μm, Allied High Tech Products, Inc.), followed by sonication in 18.2 MΩ·cm H2O for 10 min. 4.8 mg of catalyst was dispersed in a mixed solution containing 0.75 mL of 18.2 MΩ·cm H2O, 0.25 mL of 2-propanol and 50 μL of Nafion (around 5% in a mixture of water and lower aliphatic alcohols) as the binding agent. Then the suspension was sonicated for 30 min to form a homogeneous ink. After that, 5.25 μL of catalyst ink was dropped onto the GC electrode and dried under light irradiation. The catalyst loading was calculated to be 0.12 mg/cm2 in all cases of GC electrodes. Ni32Fe oxide/Ni foam electrode was fabricated by drop-casting the catalyst ink on the surface of pre-treated Ni foam and dried at room temperature. The loading was around 1 mg/cm2 by weighing the electrode before and after catalyst deposition.


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All linear scanning voltammetry (LSV) curves were collected by sweeping the potential from 0.7 to 1.7 V vs RHE with a rate of 10 mV/s. Cyclic voltammetry (CV) measurements were carried out in the potential range between 0.7 and 1.6 V vs RHE with a scan rate of 50 mV/s. The activation of catalysts was achieved by conducting continuous CV scanning. 200 CVs were applied on each sample and LSV curves were collected before and after the CVs. Stability tests were carried out using controlled current electrolysis where the potential was recorded at a constant current density of 10 mA/cm2 over a period of 50 h. The reproducibility of the electrochemical data was checked on multiple electrodes. The GC electrodes were kept rotating at a speed of 2000 rpm when measuring LSV and CV curves. In all measurements, the IR drop was compensated at 85% automatically using the potentiostat software (EC-Lab V10.44). The electrochemical impedance spectroscopy (EIS) was carried out in the same configuration with applying an anodic polarization potential of 1.6 V vs RHE on the working electrode. The spectra were collected from 105 Hz to 0.5 Hz with an amplitude of 5 mV. The ECSA was determined by measuring the non-Faradaic capacitance current associated with double-layer charging from the scan-rate dependence of CVs. CV scans with different scan rates, ranging from 60 to 180 mV s-1, were carried out in a narrow potential window from 1 to 1.1 V vs. RHE. By plotting the capacitive current (Δj = janode - jcathode) against the scan rate and fitting with a linear fit, the double layer capacitance (Cdl) can be estimated as half of the slope. The ECSA of each sample was calculated from its Cdl according to this equation: ECSA = Cdl / Cs, where Cs is the specific capacitance. In this work, 0.04 mF/cm2 is chosen as the reference value of the catalysts for OER in Fe-free 1 M KOH solution.


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The turnover frequency (TOF) was calculated by assuming that all metal atoms were catalytically active: TOF = i/(4Fn). Here, i (A) is the current measured at a specific overpotential, the number 4 represents the four electron transferred for generating 1 oxygen molecule, F is Faraday’s constant (96485.3 C/mol), and n is the moles of the metal atom based on the loading of metal oxides. The electrochemical data of STL-templated oxides are summarized in Table S4.

Results and Discussion A series of Ni-Fe oxides were prepared through a hard-templating method by using spent tea leaves (STL) as sustainable template. The fabrication of transition metal oxides through this method was recently demonstrated by our group, the key to a successful replication of hard template is a lower decomposition temperature of metal precursors, in comparison to the combustion temperature of template.39,40 Thermogravimetric analysis (TGA) was firstly conducted to study the thermal oxidation of STL in flowing air (Fig. S1). For pre-washed STL hard template, there is no significant weigh loss until reaching ~260 oC. On the other hand, the dry STL with metal precursor on the surface lost the weight dramatically since a relatively low temperature of ~150 oC, suggesting the decomposition of nickel and iron nitrate precursors. As a result, the formation of metal oxide nanoparticles took place in the pore confinement of the template before the decomposition/combustion of STL at higher temperature. It is interesting noted that the weight loss curves for STL shift to lower temperature with adding metal precursors. This can be explained by Mars-van Krevelen mechanism41-43 that the lattice oxygen reacting with carbon forms CO and oxygen vacancies which then diffuse to the surface of metal


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oxide, facilitating subsequent fracture of the C-C bond.41 As a result, the metals (e.g. Ni, Fe) with oscillable oxidation states are capable of catalyzing the thermal oxidation of carbon. Upon a slow thermal treatment applied for the crystallization of metal oxide and the following template removal, Ni-Fe oxides were obtained and their morphology was characterized by electron microscopy. As shown in the low-magnification transmission electron microscopy (TEM) images (Fig. S2), a unique structure consisting of small nanoparticles with diameters of around 8 nm was observed for NiO and Ni-Fe oxides (Ni/Fe = 64, 32, 16, 8, 4). To see more detail, the high-resolution TEM was conducted for selected Ni32Fe oxide (Fig. 1a), further demonstrating the unique structure comprising nanosized crystals. A particle size distribution was established according to this image with a mean size of 6.5 nm and narrow deviation of 0.7 nm (inset in Fig. 1a). This can be also seen from scanning electron microscopy (SEM; Fig. 1b), with these nanoparticles well packed and interconnected together. Moreover, the element mapping images obtained from the SEM image of Ni32Fe oxide (Fig. 1c-f) revealed that Ni, Fe, O elements were not localized but homogeneously dispersed over entire regions. Elemental compositions of all Ni-Fe oxides were determined by energy-dispersive X-ray (EDX) spectroscopy. As shown in Fig. S3, the relative ratio of Ni/Fe was relatively wellmatched with the theoretical ratio in all oxides even the deviation with increasing Ni content was observed by the fact that Ni oxide dissolves in acid solution more rapidly than Fe oxide,44,45 in line with the higher Ni/Fe ratio of oxide without diluted acid treatment. The presence of small amount of impurities (Mg, Al, P, Ca, Mn, etc.) in all samples was attributed from the ingredients of STL (Table S1). These impurities may have additional effects on OER, but we believe that they don’t appear to enhance the intrinsic activity dramatically.


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The textural parameters of Ni-Fe oxides, BET surface area and pore volume, were determined by nitrogen physisorption, and the values are summarized in Table S2. As seen in Fig. S4, all samples showed typical type IV isotherms, which are indicative of mesoporous materials. A similar BET surface area of around 70 m2/g was obtained and the pore volume was recorded to be in the range of 0.12 to 0.20 cm3/g, which can be supported by the uniform nanostructure from electron microscopy measurement. Such a high surface area for Ni-Fe oxides is desirable for electrochemical reaction since more active sites are exposed to contact with electrolyte. After confirming the uniform physical properties by the aforementioned characterization, X-ray diffraction (XRD) analysis was further employed to characterize the crystal structure of STL-templated Ni-Fe oxides, and their patterns are shown in Fig. 2. Distinct reflection peaks were shown on all samples, indicating these particles were highly crystalline, which is beneficial for their electrochemical stability under base condition. From the top pattern, STL-templated NiO exhibited the reflection patterns centered at 37.3o, 43.3o, 62.9o, 75.4o, 79.4o (2 theta values), representing (111), (200), (220), (311), (222) facets of NiO rock salt structure, respectively. With the incorporation of Fe into the oxides, the XRD patterns displayed characteristic reflections at the same degrees as pure NiO until increasing Fe ratio up to Ni/Fe 8/1. It has been reported that Fe has limited solubility in NiO lattice, up to 6 mol % at calcination temperature of 1200 oC.46 The iron phase such as NiFe2O4 and Fe2O3 tend to form at low calcination temperature from 200 to 550 oC.15,17,20,47 This could be verified by the XRD patterns of Ni-Fe oxides from direct calcination of metal precursors. As shown in Fig. S5, the pristine XRD patterns of Fe2O3, NiFe2O4, and NiO were emerged on non-templated Ni-Fe oxides. In contrast, no additional reflection peaks of iron phases could be observed in STL templated Ni-rich oxides with Fe


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concentration less than 20 mol %, which indicates that iron atoms are probably incorporated into the rock salt assisted by STL templating. As a result, the uniform distribution of Ni and Fe elements was confirmed in mapping images. The unusual high Fe solubility of up to 20 mol % in NiO structure was reported by Fominykh et al. which is supported with XRD, Mössbauer spectroscopy and EXAFS. This was attributed to effect of nanoscale where the metastable and defect phases are often more stable than in the bulk.20 In our case with maximum Fe solubility of ~11 mol % (in the case of Ni8Fe oxide), the nanoscale effect could apply to STL-templated NiFe oxides with particle size of around 7 to 9 nm (calculated from the XRD patterns using the Scherrer equation (Table S3)), which can be also seen from the TEM mages. Whereas sharp reflection peaks of iron phases showed in non-templated samples with much larger crystal size (Fig. S5). On the other hand, it is also possibly explained by the exothermic process of CO2 formation, where the intermediate CO generated from lattice oxygen and carbon was oxidized by gas-phase oxygen at the surface of metal oxides.41 This process could help to incorporate Fe into the crystal structure of substrate by raising the local temperature. With increasing the Fe content from 20 mol % (Ni4Fe oxide), the characteristic reflection peaks of NiFe2O4 spinel showed up at 30.3o, 35.7o and 57.4o. In the STL-templated Fe oxide, the dominant reflections belong to the structure of γ-Fe2O3 with small amount of α-Fe2O3 generated. X-ray photoelectron spectroscopy (XPS) was further used to investigate the chemical state of nickel and iron on nickel-rich oxides (Ni/Fe = 64, 32, 16, 8) which exhibited one crystal phase of NiO from XRD results. The XPS spectra of the Ni 2p region are plotted in Fig. 3a, where the peaks are almost identical for the four samples. The Ni 2p spectra comprise two regions assigned to Ni 2p1/2 (850-865 eV) and Ni 2p3/2 (870-885 eV) spin-orbit levels, ascribing to Ni2+ species in NiO.48,49 In Fig. 3b, the Fe 3p XPS regions of these Ni-Fe oxides show one


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peak at 56 eV, which is indicative of the presence of trivalent Fe atoms.50,51 In order to get more information on the oxidation state and location of Fe atoms within the oxides, Mössbauer studies were carried out on Ni-Fe oxides (Ni/Fe = 32, 4). As shown in Fig. 3c, the doublets in the spectra of Ni32Fe and Ni4Fe oxides have similar isomer shift, which is the key parameter determining the oxidation state. The values of isomer shift are 0.38 and 0.35 mm/s for Ni32Fe and Ni4Fe oxides, respectively, which are characteristic for high-spin Fe3+ ions.20,24,52 Additionally, the quadrupole splitting in Ni4Fe oxide (0.80 mm/s) is slightly smaller compared to that of Ni32Fe oxide (0.84 mm/s), suggesting a distribution of slightly more distorted geometries with higher concentration of Fe. This is in line with a distribution of iron sites, where the line width of the spectra (0.7 mm/s) is found to be ca. three times that of the spectrometer resolution (0.22 mm/s). At 4.2 K, the Mössbauer spectrum of the Ni4Fe sample in zero applied field showed spontaneous magnetic splitting (Fig. 3d), apparently due to slow magnetic relaxation of nanosized magnetic domains (which showed fast superparamagnetic relaxation at 80 K and room temperature). The magnetic spectrum could be simulated with three sextets for modelling the shoulders, particularly seen at the outer lines. Interestingly, two of the sextets, (i) and (ii), according to their internal fields of 50.7 and 54.8 T, fit remarkably well to the subspectra known for magnetic NiIIFeIII2O4 spinel, namely 51.6 T, and 55.8 T for A and B sites in bulk NiFe2O4 and in the core of nanoparticles.53 The two subspectra account for 54% of the total iron content of the Ni4Fe sample. The third subspectrum, (iii), has a remarkably high internal field at the iron nuclei of 57.7 T. This cannot easily be assigned to any of the known magnetic material. We propose a site in the interface with the NiO matrix with iron coordination similar to that in Bernalite (56 T).54 In summary, the results indicate that Fe3+ ions tend to form very small NiFe2O4


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domains/precipitates, but also may be incorporated into the rock structure of NiO, agreeing well with the XRD results where there were no other phase existed. After detailed structural analysis, the OER performances were measured following the protocol proposed by Jaramillo and co-workers.5 It has been intensively studied that Fe impurities in the KOH electrolyte can be readily incorporated into Ni(OH)2/NiOOH species. Less than 1 ppm of Fe in the electrolyte could bring a significant effect on the catalytic activity towards OER.23,35 In order to exclude the effect of Fe impurities, the electrochemical measurements were conducted in Fe-free KOH electrolyte, with all the cell components being washed with acid before experiments. As shown in the initial linear scanning voltammetry (LSV) curves (Fig. 4a), NiO was more efficient for OER in comparison with Fe2O3, and further activity enhancement was realized with incorporation of Fe. Among the mixed metal oxides, the catalytic activity showed an obvious dependence on the metal stoichiometry, where the highest OER activity was achieved on Ni-Fe oxide with a Ni/Fe ratio of 32/1. It is well-known that an activation process undergoes during water oxidation due to the surface structural changes of NiFe based catalysts. Thus, 200 cyclic voltammetry (CV) scans were applied to explore the electrocatalytic behaviors and their activation/deactivation profiles due to the surface reconstruction. Different CV curves of all samples are plotted in Fig. S6, and the oxidation peak centered at ~1.45 V vs. RHE displayed on Ni-Fe oxides is corresponded to the oxidation of Ni2+ to Ni3+. The intensity of this peak was substantially increased, supporting that the Ni-Fe oxides were activated during long-term CV measurements. For monometal oxides, a significant increase was exhibited in the broad oxidation peak of NiO, whereas the CV curves of Fe2O3 showed negligible variation. With the incorporation of Fe, the broad oxidation peak of NiO became less prominent and shifted in the positive direction (Fig. S7). This is due to the suppressing effect on


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the oxidation of Ni2+ to Ni3+ with increasing amount of Fe, which is in line with previous studies.16,17,20 The LSV curves for all samples after 200 CV scans are plotted in Fig. 4b. As shown, a similar dependence of OER activity is obtained on the metal stoichiometry, with Ni32Fe oxide being the most active catalyst.

Although the trace impurities from tea leaves were

contained in Ni32Fe oxide (Table S1), it is the same case with all the Ni-Fe oxides from tea templating which could rule out the impurity effect on activity enhancement. In order to compare the activity before and after electrochemical activation, the current densities at 1.7 V vs. RHE are summarized in Fig. 4c. Consistent with the CV results, Ni oxide and Ni-Fe oxides are activated with increased values of current density. The activity enhancement during electrochemical activation could be: (i) the formation of Ni hydroxide species on the surface when immersing NiO in KOH solution, which was further oxidized to Ni oxyhydroxide species under potential with high intrinsic OER activity, (ii) Fe incorporation into the

Ni

oxyhydroxide

species,

which

would

generate

active

metal

coordination

environments.18,55,56 The catalytic kinetics of STL-templated oxides was then investigated using Tafel plots which were directly derived from the LSV curves. The Tafel plots from initial and activated LSV curves are depicted in Fig. 4d and 4f, respectively, and the calculated Tafel slopes are summarized in Fig. 4e. Initially, the dependence of Tafel slope on metal stoichiometry matches well with that of LSV curves, with the lowest Tafel slope of 58 mV/dec for the most active sample (Ni32Fe oxide). Upon activation, the Tafel slopes of mixed Ni-Fe oxides were in a narrow range of 68-73 mV/dec, still outperforming that of monometal oxide. The independence of Tafel slope on metal stoichiometry could be as a result of the formation of active Ni-Fe oxyhydroxide species, which was on the surface of Ni-Fe oxide with electrochemical activation. Similar


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observation has also been found by Louie and Bell in a thin-film Ni-Fe oxide system, where the same Tafel slope for mixed Ni-Fe films implies a common rate-limiting step.16 Although the Fe incorporation in the conductive matrix of NiO may not alter the Tafel slope, it was reported that the current densities of Ni-Fe oxide were expected to change with Fe increasing the electrical conductivity of catalyst film.23 The electrochemical impedance spectroscopy (EIS) was carried out to assess the kinetics of electrode reaction. A good general model was proposed by Lyons and co-workers for the OER at metal oxides shown in Fig. S9.57-59 More detailed information concerning the components of the circuit is given in supporting information. As shown in Nyquist plots of activated oxides (Fig. 5a), the resistance in the high-frequency, which is related to solution resistance (RΩ), is ~7 Ω for all the oxides. By estimating the fitted data in Fig. S10, a similar influence of the metal stoichiometry was applied on the polarization resistance (Rp), as well as Rs which is related to the rate of production of surface intermediates.60 The lowest Rp and Rs for Ni32Fe oxide indicate the superior charge transfer rate and easier formation of active species for OER, respectively, which contribute to its highest catalytic activity among the oxides considering their similar surface area and active species for OER. To shed more light on the activity trend, the intrinsic activity of Ni-Fe oxide catalysts was compared by normalizing the polarization curves to the electrocatalytic active surface area (ECSA). We firstly estimated the ECSA of the catalysts by using a simple CV method in a nonFaradaic region at different scan rate (Fig. 5b), where ECSA could be represented by the linear slope (twice of the double-layer capacitance, Cdl). As depicted in Fig. S11 containing the values of Cdl and ECSA, the values for Ni oxide were higher than those for Fe oxide. Once Fe was incorporated into Ni oxide, higher Cdl and ECSA were achieved for mixed oxide, implying that


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higher OER activity of mixed oxides could be attributed to their larger ECSA compared to that of monometal oxide. It is worth to mention that the largest ECSA was obtained on the sample with lowest Fe content (Ni/Fe 64/1), instead of the most active Ni32Fe oxide, which is supported by decreased Ni2+/3+ redox charge due to the suppressing effect of Fe as aforementioned (Fig. S7).61 Next, the polarization curves for oxides were rebuilt based on their ECSA in Fig. 5c. As shown, a similar dependency of activity could be observed on metal stoichiometry. The largest normalized current density was exhibited on Ni32Fe oxide, suggesting its intrinsically highest catalytic activity among the Ni-Fe oxides. On the basis of assumption that every metal atom was involved in the catalytic reaction, the turnover frequency (TOF) was calculated at an overpotential of 350 mV to access the catalytic activity of Ni-Fe oxides normalized by the number of metal atoms on the electrode. As can be seen in Figure S12, activated Ni32Fe oxide has the highest TOF (0.0072 s-1), which is 5 times and 83 times higher than pure NiO and Fe2O3, respectively. It is worth mentioning that this TOF value is also more than 3 times higher in comparison with one benchmarked catalyst (ordered mesoporous Co3O4), with a TOF of 0.002 s-1 measured in the same system.35 In addition to Co3O4, STL-templated Ni32Fe oxide exhibited competitive OER activity compared with reported Ni-Fe oxides, which were prepared via evaporation induced self-assembly and silica hard templating methods under high temperature of 550 oC.15 At a fixed overpotential of 350 mV, a peak in activity were observed near 10 mol % Fe in both systems,15 with current density of around 3.8 and 10.5 mA/mg, respectively. In our study using tea templating to construct uniform nanoparticles, much higher current density (~37.3 mA/mg) were achieved on Ni32Fe oxide at the same overpotential. Furthermore, the specific current densities at a constant overpotential of 300 mV, which is a primary feature to define OER activity described by Louie and Bell,16 were


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collected and plotted in Fig. 5d. The specific current density of NiO is increased with increasing Fe content, reaching an optimum at a Ni/Fe ratio of 32. It was again demonstrated the strong effect of metal stoichiometry on the catalytic performance for OER. In order to investigate whether this optimal composition of nickel-iron is also valid for other synthetic approaches, a series of bulk Ni-Fe oxides were prepared through solid–solid reactions of metal nitrate precursors without using any template. The OER performances of the prepared bulk materials were measured at the same conditions. With applying 200 CV scans, similar activation behavior was also exhibited on the bulk Ni-Fe oxides (Ni/Fe = 64, 32, 16, 4) as seen in Figure S13. The maximum activity peak of bulk oxides locates at a Ni/Fe ratio of 32/1, consistent with the trend of STL-templated oxides; however, much lower current density was achieved on bulk oxides compared to that of nanostructured oxides. This is due to higher surface area from STL templating, which provides more catalytic site on the electrode. In addition, the Ni-Fe oxide with the optimal composition (32/1) was prepared under higher calcination temperature of 750 oC to study the effect on crystal phase evolution and water oxidation activity. As shown in Figure S14, distinct reflection peaks of NiO rock salt structure can be also observed from the oxide calcined under higher temperature. The narrower reflections indicate the formation of larger particles during sintering, which results in a lower surface area of Ni-Fe oxide calcined at 750 oC. The surface area was dropped from 70 to 27 m2/g, and consequently, this oxide exhibited less efficient catalytic activity towards OER compared to its counterpart calcined at 550 oC. To be employed as catalyst for practical water electrolysis, it is economically efficient to deliver a large current density at low applied potential. The loading a catalyst on the porous Ni


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foam could significantly increase the reaction rate of OER.62-64 This is due to the metallic conductivity of Ni foam which can enhance the electron transfer, and also its 3D porous structure which provides large surface area and facilitates mass transport during OER. As shown in Fig. S15, the Ni foam substrate has an interconnected macroporous structure with clean surface after a washing step in acid. The catalyst ink was dropped on the surface of Ni foam with forming a uniform oxide thin film. Therefore, the optimized catalysts, Ni32Fe oxide showing the highest OER activity on glassy carbon electrode, was deposited on Ni foam (0.5 cm2), which was then used as anode for electrochemical water splitting. The LSV curves in Fig. 6a show much higher activity of Ni32Fe oxide/Ni foam than that of Ni foam substrate, with reaching a current density of 10 mA/cm2 at 1.521 V and 1.604 V, respectively. A practical current density of 100 mA/cm2 and 500 mA/cm2 could be delivered for Ni32Fe oxide/Ni foam at a low overpotential of 356 mV and 447 mV, respectively. Such performance is comparable with benchmark transition metalbased OER catalysts (Table S5).56 The long-term stability is another important criterion to evaluate electrocatalysts for practical water electrolysis. A stability measurement was further carried out with applying a constant current density at 10 mA/cm2 continuously for 50 h (Fig. 6b). The applied potential was gradually decreased in the first hour, indicating that Ni-Fe catalyst was activated under anodic potential in KOH electrolyte. Afterwards, the potential remains nearly unchanged during the rest of measurement. In contrast, the bare Ni foam showed increasing potential for delivering such current density. It is worth noting that no detachment of catalysts took place on the electrode with vigorous oxygen evolution, when different potential were applied to drive water oxidation (Movie S1). Consequently, the oxide catalysts were wellmaintained on the surface of Ni foam after electrolysis as it is proven by the elemental mapping (Figure S16). The robust stability combined with outstanding activity to deliver high current


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density at modest potential, demonstrates that Ni32Fe oxide/Ni foam has the potential to serve as good OER catalyst for practical water electrolysis. On the basis of the aforementioned results, it could be concluded that the incorporation of Fe enhanced the OER activity of tea leaves templated Ni-Fe oxides. With a controlled morphology as well as surface area, the enhancement of activity could be explained by a combination of lower reaction resistance, larger exposed ECSA and superior intrinsic activity, due to Fe incorporation. The optimal Ni/Fe ratio was examined to be 32/1 with showing highest electrocatalytic activity. On the other hand, this templating process exhibited clear advantages of constructing nanostructure, where much higher OER activity was performed on templated Ni32Fe oxide than its bulk counterpart as a result of higher surface area (Fig. S17, 18). In addition, with the presented approach nanostructured materials can be prepared at large scale and deposited on nickel foam for a feasible electrolysis cell construction that shows promising activity and durability over 50h.

Conclusion In summary, we reported a series of nanostructured Ni-Fe oxide compositions with same textural parameters by using hard templating approach as a toolbox, where waste tea leaves were utilized as a sustainable template. The nanoparticulate oxides with controlled morphology and surface area were chosen as a model system to investigate the influence of metal stoichiometry (Ni/Fe) on the catalytic performance for electrochemical water oxidation. All the mixed oxides performed higher OER activity than either of the pure oxides, NiO and Fe2O3. The electrocatalyst with optimal composition of Ni/Fe (32/1) performed d the highest activity by possessing a small reaction resistance, a large exposed ECSA, and a superior intrinsic activity.


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For the practical application, the working electrode was further fabricated by depositing optimized Ni/Fe electrocatalyst on commercial nickel foam, where a large current density (500 mA/cm2) was realized at 1.677 VRHE and the activity was maintained over two days of applied bias. This work demonstrates that metal stoichiometry plays an essential role on the electrocatalytic performance for Ni-Fe oxides, which have great potential in the practical applications of water electrolysis.


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Figures

Fig. 1. (a) High-solution TEM image and corresponding particle size distribution (inset), (b) SEM image of STL-templated Ni32Fe oxide, (c) SEM image and corresponding elemental mapping images of (d) Fe, (e) Ni, and (f) O.


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Fig. 2. Wide-angle XRD patterns of STL-templated Ni-Fe oxides. Triangle: NiO (PDF2 entry 78-0429), star: NiFe2O4 (PDF2 entry 86-2267), square: γ-Fe2O3 (PDF2 entry 25-1402), circle: αFe2O3 (PDF2 entry 33-0664).


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Fig. 3. High resolution XPS spectra of (a) Ni 2p and (b) Fe3p for STL-templated Ni-Fe oxides. (c) Mössbauer spectra of Ni32Fe oxide and Ni4Fe oxide. (d) Mössbauer spectra of Ni4Fe oxide measured at liquid helium temperature, with green, wine and blue lines representing sextets (i), (ii), and (iii) for modeling the shoulders, respectively. The isomer shifts of subspectrum (i), (ii), and (iii) are 0.49, 0.46 and 0.54 mm/s, respectively. Quadrupole effect have not been observed, which is not unusual for oxides because the internal field may point off the principal axes of the electric field gradient tensor.54


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Fig. 4. The initial (a) and activated (b) LSV curves of STL-templated Ni-Fe oxides. (c) Summarized current density of STL-templated Ni-Fe oxides at 1.7 V vs. RHE before and after activation using 200 CV scans. The initial (d) and activated (e) Tafel plots derived from (a) and (b), respectively. (f) Tafel values of Ni-Fe oxides from STL templating.


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Fig. 5. (a) The Nyquist plots, (b) Capacitive current differences (Δj = janode - jcathode) at 1.05 vs. RHE against scan rates, (c) LSV curves normalized to the ECSA, (d) specific current density taken at 300 mV overpotential of STL-templated oxides after electrochemical activation.


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Fig. 6. (a) LSV curves of STL-templated Ni32Fe oxides loaded on Ni foam and bare Ni foam as comparison. (b) Chronopotentiometric curve of Ni/Fe 32/1@Ni foam and bare Ni foam at a current density of 10 mA cm-2.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Thermogravimetric

analysis

of

pre-washed

STL,

Fe(NO3)3·9H2O@STL,

and

Ni(NO3)2·6H2O@STL, TEM image of STL-templated Ni-Fe oxides, calculated Ni/Fe ratio of STL-templated Ni-Fe oxides from the results of EDX spectroscopy, nitrogen sorption isotherms of STL-templated Ni-Fe oxides, XRD patterns of STL-templated Ni-Fe oxides with ratio of 4/1 and 32/1, different CV curves (1st, 100th, 200th) of STL-templated Ni-Fe oxides, the LSV curves normalized to BET surface area for the STL-templated Ni-Fe oxides, equivalent circuit for the metal oxides catalyzing OER, fitted values of resistances, calculated double-layer capacitance,


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electrocatalytic active surface area and TOF at an overpotential of 350 mV of STL-templated NiFe oxides, the electrochemical result of bulk Ni-Fe oxides, XRD, BET and LSV curves of Ni32Fe oxide calcined at different temperature, SEM images of electrode fabricated on Ni foam and the electrode after electrolysis, TEM image and nitrogen-physisorption isotherm of bulk Ni32Fe oxide, the LSV curves of bulk and STL-templated Ni32Fe oxide, Elemental analysis by EDX, calculated BET surface area and pore volume, calculated crystalline domain size from Scherrer equation, and summarized electrochemical data of STL-templated Ni-Fe oxides.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by IMPRS-RECHARGE and MAXNET Energy consortium of Max Planck Society. We thank S. Palm, H. Bongard, and B. Spliethoff for EDX analysis and microscopy images. Sincere thanks to J. N. Büscher and C. Weidenthaler for XPS measurement and discussion on this part.


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Optimizing NiâFe Oxide Electrocatalysts for Oxygen Evolution

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