Optimizing Ni–Fe Oxide Electrocatalysts for Oxygen Evolution Reaction by Using Hard Templating as a Toolbox

Cite this: ACS Appl. Energy Mater. 2019, 2, 2, 1199–1209
Publication Date (Web):January 30, 2019
https://doi.org/10.1021/acsaem.8b01769
Copyright © 2019 American Chemical Society
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Abstract

A specific investigation was carried out to study the influence of the Ni/Fe ratio for oxygen evolution reaction (OER) by using the 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 a Ni/Fe ratio of 32/1. The higher catalytic performance of Ni32Fe oxide was attributed to lower reaction resistance and higher intrinsic activity, which are confirmed by electrochemical impedance spectroscopy and surface area analysis, 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

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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 multielectron 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 the water splitting process. Although noble-metal oxide electrocatalysts, such as IrO2 and RuO2, have exhibited good OER activity and are still regarded as state-of-art OER catalysts, (2,5,6) their large-scale application is not favorable in terms of their scarcity, high cost, and unsatisfying stability at high anodic potentials. 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-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 toward 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 Ni–Fe oxide materials synthesized via three different approaches, i.e., evaporation-induced self-assembly, 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 ultrasmall 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 as morphology and specific surface area have significant effects 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 that 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 the 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 nanostructures 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 a 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 advantage of the 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 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 a Ni/Fe ratio of 32. Furthermore, these electrocatalysts go through an interesting activation process. Electrochemical impedance spectroscopy and surface area analysis were conducted on STL-templated 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 the optimal ratio was further loaded on a conductive metal substrate (Ni foam) and utilized as the 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

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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 several times until there was no visible color of washing water and could be employed as template after drying in air at 90 °C. In a typical hard-templating process, 4 mmol of metal precursor was dissolved in distilled water (20 mL) with prewashed tea leaf. The mixture was then conducted at room temperature for 2 h. Nickel nitrate hexahydrate and iron nitrate nonahydrate were used as precursors and mixed with the designed molar ratio while keeping the total molar amount of metal precursors at 4 mmol. The mass ratio of prewashed tea leaf and metal precursors was controlled at 2:1 throughout the experiment. After it was dried in air at 90 °C overnight, the obtained mixture was calcined at 550 °C for 4 h with a ramping rate of 2 °C min–1. Followed by treating it in 0.1 M HCl solution (40 mL) to remove some impurities that come from the tea leaves, the final product was collected by centrifugation and repeatedly washed with distilled water (40 mL portions).

Synthesis of Nontemplated Nickel/Iron Oxide

Following the above procedure, nontemplated nickel iron oxides were synthesized by a mixture of iron and nickel precursors and were 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 H2SO4-cleaned polypropylene (PP) centrifuge tube. KOH (1 M; 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. KOH (1 M; 40 mL) was added in the tube to redisperse 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.

Materials Characterizations

Thermogravimetric (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 the 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 a reference at 284.5 eV. For Mössbauer measurements, the radioactive source for the 57Fe Mössbauer transition consists of a 5727Co 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 at 4.2 K and in the 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 MoesFit program which was 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, the latter two of which were used as the 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 before the test. The temperature of the cell was kept at 25 °C 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. Then, 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.
The Ni32Fe oxide/Ni foam electrode was fabricated by drop-casting the catalyst ink on the surface of pretreated Ni foam and was dried at room temperature. The loading was around 1 mg/cm2 by weighing the electrode before and after catalyst deposition.
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. Then, 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 LSV and CV curves were being measured. 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 = janodejcathode) 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.
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 electrons 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

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A series of Ni–Fe oxides were prepared through a hard-templating method by using spent tea leaves (STL) as a 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 first conducted to study the thermal oxidation of STL in flowing air (Figure S1). For prewashed STL hard template, there is no significant weigh loss until a temperature of ∼260 °C is reached. On the other hand, the dry STL with metal precursor on the surface lost weight dramatically after a relatively low temperature of ∼150 °C, 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 to note that the weight loss curves for STL shift to a lower temperature with adding metal precursors. This can be explained by the Mars–van Krevelen mechanism (41−43) where the lattice oxygen reacting with carbon forms CO and oxygen vacancies which then diffuse to the surface of metal 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 (Figure 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 imaging was conducted for selected Ni32Fe oxide (Figure 1a), further demonstrating the unique structure comprising the 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 Figure 1a). This can be also seen from scanning electron microscopy (SEM; Figure 1b), with these nanoparticles well packed and interconnected together. Moreover, the element mapping images obtained from the SEM image of Ni32Fe oxide (Figure 1c–f) revealed that Ni, Fe, and O elements were not localized but homogeneously dispersed over entire regions.

Figure 1

Figure 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.

Elemental compositions of all Ni–Fe oxides were determined by energy-dispersive X-ray (EDX) spectroscopy. As shown in Figure S3, the atomic ratio of Ni/Fe in Ni−Fe oxides was relatively well-matched with the stoichiometry of Fe and Ni precursors added in the hard-templating process; 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 do not appear to enhance the intrinsic activity dramatically.
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 Figure 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 Figure 2. Distinct reflection peaks were shown on all samples, indicating that 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 37.3°, 43.3°, 62.9°, 75.4°, and 79.4°(2 theta values), representing (111), (200), (220), (311), and (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 the Fe ratio was increased up to Ni/Fe 8/1. It has been reported that Fe has limited solubility in NiO lattice, up to 6 mol % at a calcination temperature of 1200 °C. (46) The iron phase, such as NiFe2O4 and Fe2O3, tend to form at low calcination temperature from 200 to 550 °C. (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 Figure S5, the pristine XRD patterns of Fe2O3, NiFe2O4, and NiO emerged on nontemplated Ni–Fe oxides. In contrast, no additional reflection peaks of iron phases could be observed in STL-templated Ni-rich oxides with an Fe 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 the nanoscale effect where the metastable and defect phases are often more stable than in the bulk. (20) In our case with a maximum Fe solubility of ∼11 mol % (in the case of Ni8Fe oxide), the nanoscale effect could apply to STL-templated Ni–Fe oxides with a 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 nontemplated samples with much larger crystal sizes (Figure 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 the 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 30.3°, 35.7°, and 57.4°. In the STL-templated Fe oxide, the dominant reflections belong to the structure of γ-Fe2O3 with a small amount of α-Fe2O3 generated.

Figure 2

Figure 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: α-Fe2O3 (PDF2 entry 33-0664).

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 Figure 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 Figure 3b, the Fe 3p XPS regions of these Ni–Fe oxides show one 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 Figure 3c, the doublets in the spectra of Ni32Fe and Ni4Fe oxides have a 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).

Figure 3

Figure 3. High-resolution XPS spectra of (a) Ni 2p and (b) Fe 3p 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 effects 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)

At 4.2 K, the Mössbauer spectrum of the Ni4Fe sample in zero applied field showed spontaneous magnetic splitting (Figure 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 modeling 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 materials. 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 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 toward 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 the experiments. As shown in the initial linear scanning voltammetry (LSV) curves (Figure 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 takes place during water oxidation due to the surface structural changes of Ni–Fe-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 Figure 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 (Figure S7). This is due to the suppressing effect on 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 Figure 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.

Figure 4

Figure 4. 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.

In order to compare the activity before and after electrochemical activation, the current densities at 1.7 V vs RHE are summarized in Figure 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, or could be (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 were 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 Figure 4d and 4f, respectively, and the calculated Tafel slopes are summarized in Figure 4e. Initially, the dependence of Tafel slope on metal stoichiometry matches well with the 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 from electrochemical activation. Similar 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 Figure S9. (57−59) More detailed information concerning the components of the circuit is given in the Supporting Information. As shown in Nyquist plots of activated oxides (Figure 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 Figure 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.

Figure 5

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

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 first estimated the ECSA of the catalysts by using a simple CV method in a non-Faradaic region at different scan rates (Figure 5b), where ECSA could be represented by the linear slope (twice of the double-layer capacitance, Cdl). As depicted in Figure S11 which contains 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, a higher Cdl and ECSA were achieved for mixed oxide, implying that higher OER activity of mixed oxides could be attributed to their larger ECSA compared to that of monometal oxide. It is worth mentioning 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 mentioned above (Figure S7). (61) Next, the polarization curves for oxides were rebuilt based on their ECSA in Figure 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 those of 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 °C. (15) At a fixed overpotential of 350 mV, a peak in activity was observed near 10 mol % Fe in both systems, (15) with a 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 densities (∼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 the OER activity described by Louie and Bell, (16) were collected and plotted in Figure 5d. The specific current density of NiO is increased with increasing Fe content, reaching an optimum value at a Ni/Fe ratio of 32. This 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, a 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, a 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 a higher calcination temperature of 750 °C 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 °C. The surface area was dropped from 70 to 27 m2/g, and consequently, this oxide exhibited less efficient catalytic activity toward OER compared to its counterpart calcined at 550 °C.
To be employed as catalyst for practical water electrolysis, it is economically efficient to deliver a large current density at low applied potential. Loading a catalyst on the porous Ni 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 Figure 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 formation of a uniform oxide thin film. Therefore, the optimized catalyst, Ni32Fe oxide showing the highest OER activity on glassy carbon electrode, was deposited on Ni foam (0.5 cm2), which was then used as the anode for electrochemical water splitting. The LSV curves in Figure 6a show that the activity of Ni32Fe oxide/Ni foam is much higher than that of Ni foam substrate, reaching a current density of 10 mA/cm2 at 1.521 and 1.604 V. A practical current density of 100 and 500 mA/cm2 could be delivered for Ni32Fe oxide/Ni foam at a low overpotential of 356 and 447 mV, respectively. Such performance is comparable with benchmark transition-metal-based 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 by applying a constant current density at 10 mA/cm2 continuously for 50 h (Figure 6b). The applied potential was gradually decreased in the first hour, indicating that Ni–Fe catalyst was activated under anodic potential in KOH electrolyte. Afterward, the potential remained 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 potentials were applied to drive water oxidation (Movie S1). Consequently, the oxide catalysts were well-maintained on the surface of Ni foam after electrolysis, as proven by the elemental mapping (Figure S16). The robust stability combined with outstanding activity to deliver high current density at modest potential demonstrates that Ni32Fe oxide/Ni foam has the potential to serve as good OER catalyst for practical water electrolysis.

Figure 6

Figure 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/[email protected] foam and bare Ni foam at a current density of 10 mA cm–2.

On the basis of the results mentioned above, 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, showing the highest electrocatalytic activity. On the other hand, this templating process exhibited clear advantages of constructing nanostructures, where a much higher OER activity was performed on templated Ni32Fe oxide than its bulk counterpart as a result of higher surface area (Figures S17 and S18). In addition, with the presented approach nanostructured materials can be prepared at a large scale and can be deposited on nickel foam for a feasible electrolysis cell construction that shows promising activity and durability over 50 h.

Conclusion

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In summary, we reported a series of nanostructured Ni–Fe oxide compositions with the same textural parameters by using the 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 of the mixed oxides performed at an OER activity that was higher than those of either of the pure oxides, NiO and Fe2O3. The electrocatalyst with optimal composition of Ni/Fe (32/1) performed with the highest activity by possessing a small reaction resistance, a large exposed ECSA, and a superior intrinsic activity. 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 2 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.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01769.

  • Thermogravimetric analysis of prewashed STL, Fe(NO3)3·9H2[email protected], and Ni(NO3)2·6H2[email protected]; 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, and 200th) of STL-templated Ni–Fe oxides; the LSV curves normalized to BET surface area for the STL-templated Ni–Fe oxides; the equivalent circuit for the metal oxides catalyzing OER; fitted values of resistances; calculated double-layer capacitance; electrocatalytic active surface area and TOF at an overpotential of 350 mV of STL-templated Ni–Fe oxides; the electrochemical result of bulk Ni–Fe oxides; XRD, BET and LSV curves of Ni32Fe oxide calcined at different temperatures; SEM images of an electrode fabricated on Ni foam and the electrode after electrolysis; TEM image and nitrogen-physisorption isotherm of bulk Ni32Fe oxide; 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 the Scherrer equation; and summarized electrochemical data of STL-templated Ni–Fe oxides (PDF)

  • Movie of the reference electrode, Pt wire (HER) cathode, and Ni32Fe [email protected] foam (OER) anode at an applied potential of 1.53–1.63 V (AVI)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Mingquan Yu - Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany
    • Gunhee Moon - Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germanyhttp://orcid.org/0000-0003-2931-1083
    • Eckhard Bill - Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germanyhttp://orcid.org/0000-0001-9138-3964
  • 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.

Acknowledgments

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This work was financially supported by the IMPRS-RECHARGE and MAXNET Energy Consortium of the Max Planck Society. We thank S. Palm, H. Bongard, and B. Spliethoff for EDX analysis and microscopy images. We also sincerely thank J. N. Büscher and C. Weidenthaler for XPS measurement and related discussions.

References

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  • Abstract

    Figure 1

    Figure 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.

    Figure 2

    Figure 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: α-Fe2O3 (PDF2 entry 33-0664).

    Figure 3

    Figure 3. High-resolution XPS spectra of (a) Ni 2p and (b) Fe 3p 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 effects 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)

    Figure 4

    Figure 4. 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.

    Figure 5

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

    Figure 6

    Figure 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/[email protected] foam and bare Ni foam at a current density of 10 mA cm–2.

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    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01769.

    • Thermogravimetric analysis of prewashed STL, Fe(NO3)3·9H2[email protected], and Ni(NO3)2·6H2[email protected]; 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, and 200th) of STL-templated Ni–Fe oxides; the LSV curves normalized to BET surface area for the STL-templated Ni–Fe oxides; the equivalent circuit for the metal oxides catalyzing OER; fitted values of resistances; calculated double-layer capacitance; electrocatalytic active surface area and TOF at an overpotential of 350 mV of STL-templated Ni–Fe oxides; the electrochemical result of bulk Ni–Fe oxides; XRD, BET and LSV curves of Ni32Fe oxide calcined at different temperatures; SEM images of an electrode fabricated on Ni foam and the electrode after electrolysis; TEM image and nitrogen-physisorption isotherm of bulk Ni32Fe oxide; 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 the Scherrer equation; and summarized electrochemical data of STL-templated Ni–Fe oxides (PDF)

    • Movie of the reference electrode, Pt wire (HER) cathode, and Ni32Fe [email protected] foam (OER) anode at an applied potential of 1.53–1.63 V (AVI)


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