Heat treatment of electrodeposited NiO films for improved catalytic water oxidation

Abstract

Recognizing the superior electrocatalytic properties of nickel oxide (NiO), we prepared cathodically electrodeposited nickel oxide (NiO) on fluorine doped tin oxide (FTO) glass substrates as binder free electrocatalysts for water oxidation. The electrodeposited nickel oxide film (NiO_(ED)) showed remarkable improvement for electrocatalytic water oxidation after heat treatment. In particular, the NiO_(ED)-400 catalyst (electrodeposited nickel oxide film heat treated at 400 °C) achieved appreciable current density ∼ 5 mA cm⁻² for the oxygen evolution reaction (OER) at the overpotential of 0.45 V vs. RHE. These catalyst films were characterized for structural, morphological, thermal and electrochemical properties, where the results reveal that the dehydration during heat treatment permanently removes the structural water with a concomitant amorphous → crystalline transformation in NiO_(ED) films, thereby making them more active catalysts for OER. In parallel investigations, nickel metal was electrodeposited on a stainless steel (SS) substrate, and was subsequently annealed in hot air to produce NiO_(HA) films at different temperatures. The NiO_(HA) films prepared by this method showed relatively high values of Tafel slopes and corresponding high overpotentials and low currents for OER, when compared to the NiO_(ED) films. Hence, simple heat treatment of the cathodically electrodeposited nickel oxide (NiO_(ED)) films showed remarkable improvements in their catalytic performances for oxygen evolution reaction, thereby making them efficient electrocatalysts for water oxidation.

---

Introduction

The economical and sustainable hydrogen production from water electrolysis has a great significance in order to shift our current fossil fuel based economy to a cleaner and renewable hydrogen energy based economy.^1–3 The significantly high mass-specific energy density, ability to be formed from renewable resources (such as wind or solar powered electrochemical water splitting) and clean nature make hydrogen the most promising fuel for the future.⁴ To achieve this goal, the development of low cost and stable electrocatalysts that can carry out efficient and facile Oxygen Evolution Reaction (OER) in water splitting hydrogen generators or Polymer Electrolyte Membrane Fuel Cells (PEMFCs) is highly desired. OER is an important energy demanding step in water electrolysis/splitting, which intrinsically in nature not favoured kinetically and requires efficient catalyst for expediting the reaction.⁵

The most promising electrocatalysts to date that display high efficiency for OER at lower overpotentials are Ir⁶ and Ru⁷ based materials and Pt group metals.⁸ However, the significantly high cost and scarcity of these materials make their large scale applications (for commercial electrolyzers) impractical. Moreover, Ru and Ir based catalysts are unstable in alkaline medium that is used in most of the commercial water electrolyzer. Recently, new catalysts for OER that are based on abundant and inexpensive transition metal alternatives such as Mn, Co, Fe and Ni have attracted growing scientific interest because some of these electrocatalysts showed significantly high current densities for OER at relatively low overpotentials.^9–12 In particular, nickel oxide (NiO) is one of the most promising catalyst for water oxidation, and is one of the most widely used co-catalyst compared to Pt or RuO₂, which has the ability to convert photocatalysts into more active water splitting catalysts.¹³ Both Ni and NiO display facile water oxidation at relatively low overpotential and that is why the Ni based alloys (such as stainless steel) are commonly used as anode materials in commercial water electrolyzer systems. NiO can be synthesized using a variety of techniques such as thermal decomposition, electron beam evaporation, electrochemical deposition (both anodic and cathodic),^14–16 chemical deposition¹⁷ and sol gel technology.¹⁸ Among these methods, electrochemical deposition is a versatile technique, as it allows control over the thickness of deposited layer simply by changing either the current or voltage.¹⁹ The major advantage of the electrochemical deposition technique is that the material can be deposited on the desired substrate without the need of binder.

Among nickel based materials, nickel oxide (NiO) is considered to be a promising electrocatalyst for efficient water electrolysis, however previously NiO has been mostly studied in the doped form or as mixed oxides for water splitting application.²⁰ The typical examples can be seen in the form of iron doped Ni (NiFe),²¹ Ni–Cu electrode,²² water oxidizing complex (WOC) of Ni, Co/Ni mixed oxides^23,24 and RuO₂/NiO type mixed oxides.²⁵ There is scarcity of work related to the studies on pure nickel oxide (NiO) and its application for catalytic water oxidation.

The pH is very important parameter for the electrodeposition of metal and metal oxide. The Pourbaix diagram of nickel (Fig. 1) shows that the equilibrium composition of different nickel oxide (NiO) phase varies with change in pH.²⁶

Fig. 1 Pourbaix diagram calculated for nickel with a concentration of 10⁻⁶ mol L⁻¹ corresponding to a concentration of 58 μg L⁻¹ nickel.²⁶

In the present work, we present a facile method to electrodeposit nickel oxide (NiO) films on the fluorine doped tin oxide (FTO) glass substrate (these nickel oxide films are labelled as NiO_(ED), where the subscript “ED” stands for electrodeposited). Though, the electrodeposition of nickel oxides (NiO_x) is reported earlier both by cathodic^27,28 and anodic deposition²⁹ techniques, to our knowledge, there is no study reported so far that explained the effect of heat treatment on the electrodeposited nickel oxide films from aqueous media for improved catalytic performance for water oxidation. Our study reveals that the electrodeposited NiO_(ED) films from aqueous media likely incorporate/take up huge amount of bound water that render the material amorphous in the as-deposited form and thereby affect their catalytic performances. Heat treatment of these electrodeposited NiO_(ED) films at temperatures (100, 200, 350 and 400 °C) showed remarkable improvements for the catalytic efficiencies of these films, making them efficient electrocatalysts for water oxidation.

In parallel developments, we also prepared NiO films by annealing (in hot air) the electrodeposited metallic Ni on the stainless steel substrates (these films are labelled as NiO_(HA), where the subscript HA stands for hot air annealed). The NiO films prepared by both methods were investigated for structural, morphological and electrochemical properties. Water splitting properties of both types of nickel oxides films ware studied and Tafel slopes were calculated. The detailed investigations of these catalyst films involve a range of analytical techniques including X-ray diffraction (XRD), thermogravimetric analysis/differential thermal analysis (TGA/DTA), scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX) and electrochemical properties such as Linear Sweep Voltammetry (LSV). The results from present investigations indicate that the permanent loss of surface water upon heat treatment was the key factor for improved catalyst performance and enhanced current densities for oxygen evolution reaction (OER). The comparison of water splitting properties of these two types of films was made, which can be useful to design the best catalyst material for catalytic water oxidation.

Experimental section

Materials

Analytical grade nickel chloride (NiCl₂·6H₂O), nickel sulphate (NiSO₄·6H₂O), ethylenediaminetetraacetic acid (EDTA), potassium chloride (KCl) and potassium hydroxide (KOH) were purchased from Aldrich and were used as received without any further purification. Ultra-pure deionized water was used for making up the electrolyte solution for electrodeposition.

Electrodeposition of nickel oxide (NiO) films

Scheme 1. Thin films of nickel oxide (NiO_(ED)) were electrodeposited from an aqueous solution of 0.5 M nickel chloride containing 0.1 M KCl as supporting electrolyte. In order to electrodeposit NiO, Pourbaix diagram (Fig. 1) shows that the pH of electrolyte solution should be ≥8. Hence, the pH of solution was maintained at 8 by adding appropriate amount of KOH. Since, Ni²⁺ can easily precipitate out as nickel hydroxide at this pH, approx. 0.4–0.5 M ethylenediaminetetraacetic acid (EDTA) was used as complexing agent to avoid the hydroxide formation. Deposition temperature was maintained at 20–30 °C. A higher temperature than this range showed formation of metallic nickel layer in addition to the NiO. Nickel oxide was electrodeposited on the fluorine doped tin oxide (FTO) glass substrate having a sheet resistance of 15 Ω. Prior to deposition, the FTO glass was ultra-sonicated for 30 min in the dilute solution of a commercial detergent for degreasing. After degreasing, the FTO glass was thoroughly washed with deionized water and then ultra-sonicated again in ethanol to remove any residual detergents left over on the surface of FTO. A graphite rod was used as counter electrode, while Ag/AgCl (saturated) was used as reference electrode. The nickel oxide thin films were electrodeposited at 25 °C for 30 min under galvanostatic conditions using a current density of −2 mA cm⁻². The NiO_(ED) films prepared this way were studied in the as-deposited form and were heated at 100, 200, 350 and 400 °C, which were labelled as NiO_(ED), NiO_(ED)-100, NiO_(ED)-200, NiO_(ED)-350 and NiO_(ED)-400 respectively (the subscript ED stands for electrodeposited). All electrochemical deposition experiments were performed on Gamry Reference 750 galvanostat/potentiostat system.

Scheme 2. In the second method, we first electrodeposited a thin layer of nickel (metal) on the Stainless Steel (SS) substrate. In these deposition experiments, SS was used as a working electrode, Ni rod as counter electrode, while Ag/AgCl was used as the reference electrode. The bath solution contained 0.5 M NiSO₄ and 2 M NaCl. The pH of solution was maintained at about 4 by adding boric acid (H₃BO₃). The bath was operated at 50–60 °C under galvanostatic conditions using a current density of −10 mA cm⁻². The SS substrate was degreased by treating with dilute alkali solution containing a mixture of NaOH and Na₂CO₃ and washed thoroughly with hot deionized water prior to be subjected for deposition. Once the metallic Ni was electrodeposited, heat treatment was done at 600, 700, 800 and 900 °C. In a typical heat treatment method, the Ni plated SS samples were heated to the desired temperature at a ramp rate of 10 °C min⁻¹ after which the samples were kept at the aforementioned temperatures for 2 hours in a muffle furnace under continuous air flow to convert this metallic nickel into nickel oxide. The nickel oxide films prepared by hot air annealing of Ni metal layer were labelled as NiO_(HA)-600, NiO_(HA)-700, NiO_(HA)-800 and NiO_(HA)-900 respectively (the subscript HA stands for hot air annealed).

Material characterization

The morphology and chemical composition of the nickel oxide films were investigated using JEOL JSM-6490A scanning electron microscope (SEM) that was equipped with energy dispersive X-ray analyzer (EDX). The crystal structure was investigated using STOE θ–θ X-rays diffractometer within 10–80 (2θ degrees) range using CuKα radiations (wavelength 1.543 Å). In order to obtain the IR spectrum, the scratched powder was mixed with KBr to prepare pellets. The IR spectrum was recorded in the spectral range of 400–4000 cm⁻¹ using Perkin Elmer spectrometer of Spectrum-100 model.

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the electrodeposited nickel oxide films (NiO_(ED)) films were performed on the Perkin Elmer Diamond-II equipment.

Electrochemical measurements

All electrochemical experiments were performed on VSP Biologic potentiostat/galvanostat 350 system using standard three electrode configuration. The cyclic voltammetry (CV) experiments for electrodeposition of nickel oxide films were carried out on an exposed active surface area (1 cm²) of the FTO substrate that was used as the working electrode. The platinum gauze was used as the counter electrode, while saturated silver/silver chloride (Ag/AgCl) was used as reference electrode.

For catalytic water oxidation on NiO films, the three electrode system consist of electrodeposited NiO films on FTO substrate (NiO_(ED) films) and steel substrates (NiO_(HA) films) as working electrode, Ti foil as the counter electrode and standard calomel electrode (SCE) as the reference electrode. The catalytic water oxidation reactions were carried out in an alkaline solution of 1 M NaOH. The Linear Scan Voltammetry (LSVs) were recorded for the oxygen evolution reaction (OER) by scanning from 0 to 1 V in oxidation window vs. SCE at a scan rate of 5 mV s⁻¹. The potential axis was rescaled for Reference Hydrogen Electrode (RHE) using the relation as E (RHE) = E (SCE) + 0.059 × pH + 0.241.³⁰ The overpotential (η) is commonly expressed in terms of oxygen overpotential that was determined using the relation η (mV) = [E (V) vs. SCE − (1.23 − 0.059 × pH) + 0.241] × 1000.

Results and discussion

Electrodeposition of nickel oxide (NiO) films

Cyclic voltammetry (CV) was performed to cathodically electrodeposit nickel oxide (NiO) thin films onto the surface of FTO glass (Fig. 2). The pH of the bath solution (0.5 M nickel chloride containing 0.1 M KCl as supporting electrolyte in water) was maintained at 8 by adding appropriate amount of KOH. Since, Ni²⁺ can easily precipitate out as nickel hydroxide at this pH, EDTA (0.4–0.5 M) was used as complexing agent to avoid the precipitation.

Fig. 2 The cyclic voltammogram of the complexed nickel solution (0.5 M NiCl₂·6H₂O + 0.1 M KCl + 0.4 M EDTA) on FTO glass.

The cyclic voltammogram (CV) traces were recorded within the range of +0.3 to −0.8 V vs. Ag/AgCl_(saturated) at the scan rate of 50 mV s⁻¹. In the cathodic scan, the first reduction peak observed at an onset of ca. −0.24 V can be attributed to the electrochemical reduction of (Ni²⁺ → Ni).³¹ The second reduction phenomenon observed at around −0.3 V corresponds to the electrochemical reduction of water to hydroxide, which subsequently formed nickel hydroxide Ni(OH)₂ (as per eqn (1)) and finally end up in the formation of nickel oxide (NiO) (as per eqn (2)).³¹ The final reduction phenomenon observed at above −0.5 V onwards can be attributed to the electrochemical reduction of water at the surface of NiO_(ED) thin film.

NiCl₂·6H₂O + H₂O → Ni(OH)₂ + 2HCl + 5H₂O (1)

Ni(OH)₂ → NiO + H₂O (2)

From Fig. 2, we can see that the oxidation of metallic nickel layer (Ni → Ni²⁺) could give rise to a peak during the anodic scan at about 0.2 V,²⁸ where a metallic Ni layer was initially deposited during the cathodic scan (Ni²⁺ → Ni). A. C. Sonavane and co-workers have reported the Ni²⁺ → Ni reduction during the cathodic electrodeposition of nickel oxide on FTO glass.³¹ However, the authors found large reduction currents for this phenomenon, which caused significant amount of metallic nickel deposited onto the surface of FTO substrate. In our study, we found that when the amount of EDTA (complexing agent) was kept less than 0.3 M against 0.5 M NiCl₂·6H₂O solution, a thick layer of metallic nickel was deposited along with nickel oxide. Hence, we used 0.4–0.5 M EDTA for complexing the Ni²⁺ ions in the bath solution (0.5 M nickel chloride containing 0.1 M KCl as supporting electrolyte in water) that was used for electrodeposition. In contrast to the previous report on NiO deposition,³¹ we found that using large amount of EDTA helped to reduce the deposition of metallic nickel layer, which was evidenced from the small reduction currents for Ni²⁺ → Ni phenomenon in the CV diagram (Fig. 2). Hence, by using the high concentration of nickel solution (0.5 M NiCl₂·6H₂O) and high concentration of complexing agent (0.4–0.5 M EDTA), there was still a large amount of Ni²⁺ ions, which were complexed by OH ions produced during the subsequent reduction phenomenon. There were few more observations for the presence of Ni²⁺ ions after the first reduction phenomenon of Ni²⁺ → Ni;

(1) The colour of Ni-EDTA complexed solution didn't fade after the first reduction (Ni²⁺ → Ni) and remained deep blue, indicating the complex solution of Ni²⁺ with EDTA.

(2) There was no distinct fading of the blue coloured Ni-EDTA complex solution even after continuous 30 min galvanostatic electrodeposition.

Structural characterization

The structural analysis of both types of nickel oxide films (NiO_(ED)) and (NiO_(HA)) were carried out by X-ray diffraction (XRD) technique. Fig. 3 shows the XRD patterns of FTO glass, electrodeposited (NiO_(ED)) and heat treated (NiO_(ED)-100, NiO_(ED)-200, NiO_(ED)-350 and NiO_(ED)-400) films. The corresponding planes (hkl) and observed d-values confirm the presence of crystalline nickel oxide for samples that were annealed at 350 °C and above this temperature. Similar XRD patterns were reported earlier for nickel oxides.^32,33 The peaks obtained cantered around (ca.) 37° 2θ (111), 43° 2θ (200), and 62.8° 2θ (220) confirms the cubic NiO (ICSD#01-073-1523), which agrees well with the standard values.³⁴ No X-rays diffraction (XRD) peaks corresponding to NiO were observed in the as-deposited films neither in the films heated at 100 °C and 200 °C. These results show that the freshly electrodeposited films were amorphous in nature and crystallinity was achieved in the films that were annealed at temperatures ≥ 350 °C. The supporting electrolyte (KCl) peaks were denoted by oval in Fig. 3. Later on, the TGA/DTA data confirm the amorphous → crystalline transition in the NiO_(ED) films that occurs at temperatures ≥ 350 °C. The electrodeposited NiO_(ED) films and the films that were subjected to heat treatment at 100 °C and 200 °C are deemed to be amorphous in nature and the presence of bound water might stop the nickel oxide phase to achieve the true crystalline phases in the as-deposited thin films (NiO_(ED)) or in the films that were heat treated at 100 °C (NiO_(ED)-100) and 200 °C (NiO_(ED)-200).³⁴

Fig. 3 X-rays diffraction (XRD) patterns recorded for electrodeposited NiO films on the FTO substrate viz. NiO_(ED), NiO_(ED)-100, NiO_(ED)-200, NiO_(ED)-350 and NiO_(ED)-400.

In a visual examination, the electrodeposited NiO_(ED) films were green in colour and a colour change from green to black was observed when NiO_(ED) films were heat treated at the temperatures ∼ 350 °C and 400 °C. This black colour is attributed to the non-stoichiometric NiO (indicating that the Ni : O ratio deviates from 1 : 1).³⁵ Typically, the stoichiometrically correct NiO is green in colour (Ni : O ratio of 1 : 1) as was observed in the electrodeposited NiO_(ED) films, while the colour change from green to black in NiO_(ED)-400 indicate towards the development of non-stoichiometric NiO after heat treatment at this temperature.

Fig. 4(a) displays the XRD patterns of electrodeposited nickel metal (Ni) and the nickel oxide films formed by the heat treatment of the electrodeposited nickel metal at 600, 700 and 800 °C. The Fig. 4(b) shows the XRD pattern of nickel oxide formed by the same method at 900 °C. The XRD peaks ca. 37°, 43° and 62.8° 2θ exhibit the presence of cubic nickel oxide (ICSD#01-073-1523) phase along with the peaks due to the metallic fcc nickel (ICSD#01-070-1849) just above 44°, 51.8° and 76.3° 2θ due to (111), (200) and (220) lattice planes respectively.³⁶

Fig. 4 (a). XRD patterns recorded for electrodeposited metallic Ni, NiO_(HA)-600, NiO_(HA)-700 and NiO_(HA)-800, (b) XRD pattern of electrodeposited NiO_(HA)-900.

The electrodeposited nickel (metal) layer had a silvery lustre. Heat treatment at 600 °C and 700 °C changed the appearance of the metallic Ni to light green (suggesting the formation of NiO). Upon further heat treatment of this film till 800 °C caused a colour change from green to black, which was due to the formation of composite electrode (nickel oxide and iron oxide) as shown in the XRD pattern of the NiO_(HA)-800 in Fig. 4(a). The XRD peaks generated due to the formation of iron oxides (ICSD#01-089-0598) in NiO_(HA)-800 sample is represented by oval shape in Fig. 4(a). At temperature as high as 900 °C, the reaction between electroplated nickel and underneath iron from stainless steel caused the formation of nickel ferrite, as can be seen from the XRD pattern of NiO_(HA)-900 (Fig. 4(b)) (ICSD#067846). While heating the NiO films from 600 °C to 800 °C, an increase in the peak intensity corresponding to nickel oxide films, while a concomitant decrease in the peak intensity for metallic nickel can also be seen at the same time. It was also observed that the electrodeposited metallic Ni layer on mild steel substrate does not change to nickel oxide, if heated below 600 °C. The XRD analysis shows only peaks from metallic Ni for the samples heated below 600 °C. Therefore, 600 °C is the minimum temperature required to convert the electroplated metallic nickel into nickel oxide (NiO).

Thermal analysis

Fig. 5 shows the TGA and DTA traces of the electrodeposited nickel oxide (NiO_(ED)) film. The TGA curve (blue) shows that below 400 °C, the weight loss (%) occurred in two major steps. The first weight loss step in TGA (spanned from 60 °C to 200 °C) can be correlated to the first endothermic peak on DTA, which indicate towards the removal of structural water present in the as-deposited films of NiO_(ED). A study by Zhou, Macfarlane and co-workers showed that the heat treatment of manganese oxide (MnO_x) films electrodeposited from aqueous electrolytes show remarkable improvement in catalytic water oxidation, primarily due to the loss of structural water.³⁷ A few other metal oxides also showed similar effects of heat treatment as reported earlier.³⁸ Fig. 5 shows that the wt loss (%) due to the removal of bound water was around 13% upto 200 °C. Earlier, it has been reported that a characteristics wt loss due to the removal of water from electrodeposited nickel hydroxide (Ni(OH)₂) occurs around 250–350 °C that accompanies endothermic peak.³⁸ However, in our work, the second weight loss peak on TGA trace started above 350 °C, and was accompanied by the corresponding exothermic peak in DTA curve. This wt loss peak in TGA along with the exothermic peak in DTA can be attributed to the removal of water from nickel oxide (NiO_(ED)) with a concomitant phase transformation, as was observed in the case of amorphous → crystalline transition of CeO_x.³⁹ The TGA/DTA results therefore suggest the likely presence of amorphous nickel oxide along with the bound water in the electrodeposited (NiO_(ED)) film, while the presence of amorphous (Ni(OH)₂) cannot be excluded just on the basis of these TGA/DTA results. In the next section, FTIR results throw more light on the presence of both nickel oxide (NiO) and nickel hydroxide (Ni(OH)₂) in addition to bound water in these electrodeposited (NiO_(ED)) catalyst films.

Fig. 5 TGA and DTA traces of the electrodeposited nickel oxide (NiO_(ED)) film. The NiO_(ED) films was scratched from the FTO substrate, and was loaded into the aluminium pan that was hermetically sealed prior to run TGA/DTA experiment.

FTIR analysis

Fig. 6 shows the FTIR spectra of electrodeposited nickel oxide (NiO_(ED)) and heat treated films (NiO_(ED)-100, NiO_(ED)-200, NiO_(ED)-350 and NiO_(ED)-400) within the frequency range of 400–4000 cm⁻¹. We can see that the NiO_(ED) film didn't show sharp intense bands compared to NiO_(ED)-100, NiO_(ED)-200, which could be due to the presence of bulk water that was randomly distributed in the freshly electrodeposited nickel oxide (NiO_(ED)) film. The only IR peaks that were fairly observable in the NiO_(ED) film (3435 and 1594 cm⁻¹) were mainly from the water molecules. At this point, it is important to correlate the behaviour of the electrodeposited NiO_(ED) film with the heat treated films in the light of various techniques used in this study. The thermal studies using TGA/DTA (Fig. 5) suggest that the NiO_(ED) film contained the maximum amount of water compared to the heat treated films, while the morphology studies using SEM (Fig. 7) shows no clear morphology for this film. These observations on the (NiO_(ED)) were finally reflected by poor catalytic performance of this film, where the Linear Sweep Voltammetry (LSV) plot (Fig. 9(a)) shows the lowest current densities for water oxidation achieved by this film. In contrast, the FTIR spectra of NiO_(ED)-100, NiO_(ED)-200 show several intensive bands at different frequencies viz. 453, 663, 1111, 1404, 1594 and 3435 cm⁻¹. The broad peak observed at 3435 cm⁻¹ in the FTIR spectra of these films can be attributed to the stretching vibration of the hydrogen bonded water, while the intense peak at 1594 cm⁻¹ corresponds to the bending vibration of water in these films. The high intensity of the –OH group peaks in NiO_(ED)-100, NiO_(ED)-200 films suggest that the heat treatment improved the hydrogen bonding networks in these films. It seems that the left over water in these films adopted some order, which could be more likely the water of crystallisation. Another important peak in these films around 453 cm⁻¹ can be assigned to the stretching vibration of nickel to oxygen bond,⁴⁰ which could be due to either nickel hydroxide or nickel oxide phases. The hydroxide phase (Ni(OH)₂) was also confirmed by the peak at 667 cm⁻¹.⁴¹ Other IR peaks confirm the presence of chloride and hydroxyl ions present in these films.

Fig. 6 FTIR spectra of NiO_(ED), NiO_(ED)-100, NiO_(ED)-200 and NiO_(ED)-350 and NiO_(ED)-400 recorded over the frequency range of 400–4000 cm⁻¹.

Fig. 7 SEM images of electrodeposited nickel oxide (NiO_(ED)) and the heat treated nickel oxide films (NiO_(ED)-100, NiO_(ED)-200, NiO_(ED)-350 and NiO_(ED)-400).

As a matter of fact, the FTIR spectrum didn't give enough information on how much nickel hydroxide was converted to nickel oxide with the increase in annealing temperature from 100 °C to 400 °C. Fig. 6 shows that there was no observable difference in the FTIR spectra of NiO_(ED)-350 and NiO_(ED)-400 catalyst films, beside the fact that the amorphous → crystalline phase transition occur at temperatures ≥ 350 °C. The FTIR peak for nickel to oxygen bond, which appeared at 453 cm⁻¹ in NiO_(ED)-100 and NiO_(ED)-200 samples couldn't be observed in NiO_(ED)-350 and NiO_(ED)-400 films. However, the XRD analysis (Fig. 3) described above clearly indicates the emergence of NiO (111) and NiO (200) phases in the NiO_(ED)-350 and NiO_(ED)-400 catalyst films. We can also see that the IR spectra for NiO_(ED)-350 and NiO_(ED)-400 exhibited relatively low intensity for –OH groups stretching and bending vibration (3435 cm⁻¹ and 1594 cm⁻¹), which can be explained by the loss of water (as shows by TGA analysis (Fig. 5)) and transformation of Ni(OH)₂ to NiO as shown by XRD studies (Fig. 3).

In summary, the IR spectra reveal that the NiO_(ED), NiO_(ED)-100 and NiO_(ED)-200 contain significant amount of water compared to NiO_(ED)-350 and NiO_(ED)-400 samples. The improved hydrogen bonding network in NiO_(ED)-100 and NiO_(ED)-200 films by heat treatment indicate that the remaining water has adopted some order, more likely in the form of water of crystallization. In short, the FTIR study could only help to differentiate the amount and nature of water in these films. This technique was not found enough sensitive to clearly identify the presence of amorphous and crystalline phases in these catalyst films.

Microstructure of NiO films

Fig. 7 shows the scanning electron microscopy (SEM) images of the thin films of electrodeposited nickel oxide (NiO_(ED)) and heat treated nickel oxide films viz. NiO_(ED)-100, NiO_(ED)-200 and NiO_(ED)-350 and NiO_(ED)-400. We can see that the NiO_(ED) film couldn't achieve a clear morphology, probably due to the presence of large quantity of bound water in this material. However, the cracks can be seen in this film, as the NiO_(ED) was dried before taking the SEM images. This cracked nature of the NiO_(ED) film is attributed to the contraction upon drying due to the tensile stress. Earlier, it has been reported that the films with thickness above 0.2 micron usually show cracked structure.⁴² Similar observation was reported earlier for electrochemically deposited thin films of zirconia⁴³ and molybdenum oxide that were electrodeposited under galvanostatic conditions.⁴⁴ A granular and dense morphology was observed for heat-treated thin films. The NiO_(ED)-100 film exhibited a loose morphology with granular shaped nickel oxide particles. This kind of loose structure provides high surface area for catalytic water splitting.⁴⁵ Agglomeration of particles was observed for the films of NiO_(ED)-200 and NiO_(ED)-350. This agglomeration became further dense upon heat treatment of these nickel oxide films. The NiO_(ED)-400 film contains bulk of crystalline nickel oxide comparing to other films thereby showing somewhat different morphology.

For the nickel oxide (NiO_(HA)) films that were prepared via high temperature annealing of electroplated nickel metal, clear morphological changes were observed upon annealing from 600 °C to 900 °C (Fig. 8). The changes in morphology were probably due to the different phases present in the films, which have their origin in different annealing temperatures. The presence of other phases in NiO_(HA)-800 and NiO_(HA)-900 was also indicated by the XRD patterns of these films (Fig. 4(a) and (b)).

Fig. 8 SEM images of NiO_(HA)-600, NiO_(HA)-700, NiO_(HA)-800 and NiO_(HA)-900 films prepared by hot air annealing of electrodeposited nickel film on stainless steel substrate.

Further confirmation for the presence of metallic nickel in the NiO_(HA)-600, NiO_(HA)-700 and NiO_(HA)-800 is evident from the EDX analysis of these nickel oxide films, while the presence of iron oxide in addition to nickel oxide was also confirmed by the EDX analysis of NiO_(HA)-800 and NiO_(HA)-900 films. The EDX analysis supported the XRD results in identifying the presence of other impurities in these NiO_(HA) films.

Electrochemical analysis

The catalytic activities of the electrodeposited NiO_(ED) films were examined by linear scan voltammetry (LSV) in 1 M NaOH_(aq.) at the scan rate of 5 mV s⁻¹. Fig. 9 shows that the thin film of NiO_(ED)-400 catalyst (prepared by the heat treatment of electrodeposited NiO_(ED) films at 400 °C) achieved appreciable current density ∼ 5 mA cm⁻² at the overpotential of 0.45 V vs. RHE for water oxidation. On the other hand, the as-deposited NiO_(ED) film could generate only small currents. We can expect this kind of behaviour, since the as-deposited NiO_(ED) film was highly amorphous with significant amount of bound water in it. In contrast to the as deposited film, the heat treated nickel oxide films viz. NiO_(ED)-100 and NiO_(ED)-200 films showed improvements in currents densities for water oxidation (Fig. 9). It is interesting to note that the XRD patterns of these films (Fig. 3) didn't show any peaks that indicate the presence of crystalline nickel oxide. However, if we consult the TGA/DTA results, we can see that these films did loose water upon heating. Hence, regardless of the fact that NiO_(ED)-100 and NiO_(ED)-200 films didn't exhibit any improvement in the crystallinity (based on XRD results), the improved current densities observed for these films indicate that the removal of water by heat treatment had a positive effect on the catalytic performances of these electrocatalyst NiO_(ED) films. Previously, D. R. MacFarlane and co-workers reported similar observations of improved catalytic performances for manganese oxides films (MnO_x)⁴⁶ after heat treatment. Nevertheless, crystallinity always is not a direct indicative of the better catalytic performance. In past, catalytic activity of cobalt oxides (CoO_x) films was found to decrease with increase in crystallinity of the material.⁴⁵ Fig. 9(a) also shows that the NiO_(ED)-100 and NiO_(ED)-200 achieved less current densities for OER than NiO_(ED)-350 and NiO_(ED)-400 beside the fact that the value of Tafel slopes for NiO_(ED)-100 was 96 mV per decade and for NiO_(ED)-200 was 135.7, which were lower than 210 mV per decade for NiO_(ED)-350 and 136.4 mV per decade for NiO_(ED)-400. In principle, a lowering in the Tafel slope value is an indicative of the better catalytic activity of any catalyst materials. Though, the NiO_(ED)-100 and NiO_(ED)-200 showed relatively low values of Tafel slope, the occurrence of water in these catalyst films (shown by TGA/DTA) might be responsible for their loose amorphous structure and such materials often undergo mechanical degradation, when electrochemical reactions take place at the catalyst surface. Method of preparation and durability of material are also found to affect the electrochemical properties of many water oxidation catalysts.⁸

Fig. 9 (a) Linear Sweep Voltammetry (LSV) and (b) Tafel plots of electrochemically as-deposited and heat treated nickel oxide (NiO_(ED)) films in 1 M NaOH at the scan rate of 5 mV s⁻¹. The subscript “ED” stands for electrodeposited samples.

The materials prepared at high temperatures usually posses better crystallinity, less defective structure and corresponding higher catalytic activity compared to the amorphous ones that are often obtained at low temperature.^8,47,48 Fig. 9(b) shows that the NiO_(ED)-400 film prepared at temperature above amorphous → crystalline phase transition exhibited a Tafel slope value ∼ 136.7 mV per decade. This value of Tafel slope was close to that for NiO_(ED)-200 and even higher than that for as synthesized NiO_(ED) film. A close inspection of Fig. 9(a) shows that the onset value for OER was lowest in the case of NiO_(ED)-400 and the same material achieved the highest current density for OER among all other samples. It seems that the crystalline NiO_(ED)-400 film exhibits the best catalytic performance among all other electrodeposited nickel oxide films.

A closer look at the LSVs of these NiO_(ED) films in Fig. 9(a), we can see a small oxidation phenomenon at or just above 1.4 V vs. RHE. Previously, the water oxidation studies of nickel oxide films suggest that this phenomenon corresponds to the oxidation of Ni²⁺ to Ni³⁺ in alkaline media and can be indicated by the following equation.³¹

NiO + xOH¹⁻ → NiOOH + xe⁻¹

By observing the physical appearance of the electrodeposited nickel oxide films, we found that the NiO_(ED)-100 and NiO_(ED)-200 films were light green in colour initially, which later converted to brownish grey, when these electrodes were subjected to water oxidation in 1 M NaOH_(aq.) solution. Previously, it is reported that this brownish grey colour is due to the formation of nickel oxy hydroxide (NiOOH),³¹ which is a highly conductive material and contains significantly high number of active sites for OER.^49–51 In contrast to as-deposited NiO_(ED), NiO_(ED)-100 and NiO_(ED)-200 films, the NiO_(ED)-350 and NiO_(ED)-400 were already dark grey in colour and hence no clear change in colour was observed during OER; the high current density achieved by these high temperature NiO films can therefore be attributed to the water removal and high crystallinity of these catalyst materials. In addition, the possible formation of NiOOH during water electrolysis and its contribution towards OER can't be ignored for NiO_(ED)-350 and NiO_(ED)-400 films. Further studies are required to investigate the current generated due to the oxidation of Ni²⁺ to Ni³⁺ with the possible formation of NiOOH and its contribution in the OER for these NiO electrocatalysts films.

Revisiting the LSVs in Fig. 9(a), the second onset of potential above 1.6 V vs. RHE can be assigned to the OER, which agreed well with the previously reported values for OER using NiO electrocatalysts films.⁵² Here, we can see that the NiO_(ED)-400 catalyst (prepared by the heat treatment of as-deposited NiO_(ED) films at 400 °C) achieved appreciable current density ∼ 5 mA cm⁻² at the overpotential of 0.45 V vs. RHE for OER.

In parallel investigations, we also studied the electrochemical behaviour of as deposited Ni (metal) and nickel oxide (NiO) films, where the NiO films were formed by the oxidation of initially deposited Ni layer on stainless steel (SS) substrate. Fig. 10(a) shows the LSVs and Fig. 10(b) shows the corresponding Tafel plots of electroplated nickel (Ni) metal and nickel oxide films prepared by the heat treatment of electrode plated metallic Ni layer at 600, 700, 800 and 900 °C. Fig. 10(a) shows that the electrode with metallic Ni layer achieved the highest current density with lowest overpotential for OER, which could be due to the excellent corrosion resistance properties of Ni metal itself in alkaline media. We can also see that the nickel oxides viz. NiO_(HA)-600, NiO_(HA)-700, NiO_(HA)-800 and NiO_(HA)-900 display low current density values for OER compared to the metallic Ni. Among these nickel oxide based electrocatalysts films, the NiO_(HA)-900 exhibited the highest value of overpotential and lowest current density, which could be due to the presence of significant amount of iron oxide in NiO_(HA)-900 sample.

Fig. 10 (a) Linear Sweep Voltammetry (LSV) and (b) Tafel plots of pure nickel (Ni) and nickel oxide (NiO_(HA)) films in 1 M NaOH_(aq.) at the scan rate of 5 mV s⁻¹. The subscript “HA” stands for hot air annealed samples.

In contrast, NiO_(HA)-600 showed lowest overpotential among other oxides, while NiO_(HA)-800 displayed the lowest Tafel slope value. It was worth noticeable that the NiO_(HA)-800, which contained nickel oxide as the major fraction (as indicated by XRD analysis) still showed high overpotential and low current density for OER (Fig. 10(a)). If we compare these results with NiO_(HA)-900, it can be concluded that the inclusion of iron oxide has a negative impact on the electrocatalytic properties of these NiO based catalyst films for OER. As the concentration of iron oxide increases (such as in NiO_(HA)-900), the overpotential for OER also increases accordingly. These results also indicate that the iron oxide phases formed in this way are poor catalyst materials compared to the pure nickel oxides phases for oxygen evolution reaction (OER).

Another important observation in the LSVs of the metallic nickel and the nickel oxides (prepared by oxidation of Ni) shown in Fig. 10(a) is a single oxidation phenomenon, which is more likely the OER. The first oxidation phenomenon observed in the nickel oxide films (NiO_(ED), NiO_(ED)-100, NiO_(ED)-200, NiO_(ED)-350 and NiO_(ED)-400) that corresponds to the oxidation of Ni²⁺ to Ni³⁺ in alkaline media was not observed in the LSVs of metallic nickel (Ni) and the nickel oxide films (NiO_(HA)-600, NiO_(HA)-700, NiO_(HA)-800 and NiO_(HA)-900). To our understanding, the absence of first oxidation phenomenon in the later cases suggests that the nickel oxides prepared by the heat treatment of metallic nickel is not pure in nature but a mixture of pure metal and metal oxides. The oxidation of Ni²⁺ to Ni³⁺ and the concomitant formation of nickel oxy hydroxide (NiOOH), which was found to accelerate the oxygen evolution reaction (OER) in case of pure nickel oxides (formed by Scheme 1) seem to be absent in these NiO_(HA) films (formed by Scheme 2). A plausible explanation for the absence of this oxidation process is that the water oxidation at metallic nickel dominates the other electrochemical phenomena (water oxidation at nickel oxide surface), which can be evidenced from the fact that all electrocatalysts films prepared by Ni oxidation (NiO_(HA)-600, NiO_(HA)-700, NiO_(HA)-8000 and NiO_(HA)-900) formed from the oxidation of metallic nickel layer followed the same trend as shown by metallic nickel film itself (Fig. 10(a)). Since, metallic nickel was present in almost all these nickel oxide films, the dominant process of OER at the metallic nickel might inhibit the oxidation of Ni²⁺ to Ni³⁺ in the nickel oxide fraction of these films.

Conclusions

Herein, we prepared nickel oxide (NiO_(ED)) films on the FTO substrate by cathodic electrodeposition in aqueous electrolyte media. The NiO_(ED) films after facile heat treatment showed significant improvements for catalytic water oxidation. In particular, the NiO_(ED)-400 film achieved appreciable current density ∼ 5 mA cm⁻² for oxygen evolution reaction (OER) at the overpotential of 0.45 V vs. RHE. The electrocatalyst films NiO_(ED)-100 and NiO_(ED)-200 contain significant quantity of nickel hydroxide and therefore showed poor catalytic activities for water oxidation. In contrast, the NiO_(ED)-350 and NiO_(ED)-400 films, which contained more nickel oxide fraction compared to the hydroxide phase exhibited improved catalytic performances for OER. The detailed investigations of the physiochemical and electrochemical properties of these electrodeposited and heat treated nickel oxide films reveal that the heat treatment primarily caused the permanent loss of structural water along with amorphous → crystalline transformation in these NiO_(ED) films, especially when heated at temperatures ≥ 350 °C. In parallel development, we also prepared the NiO_(HA) films by first electroplating metallic nickel layer on a stainless-steel substrate, followed by the annealing of electroplated Ni layer in hot air. The results reveal that the NiO_(HA) films formed by this method were impure in nature and consist of a mixture of Ni metal and mixed metal oxides. The NiO_(HA) films formed this way showed higher overpotentials, indicating poor catalytic performances of these catalyst films for water oxidation.

Acknowledgements

U. A. Rana would like to extend his sincere appreciation to the Deanship of Scientific Research at the King Saud University for its funding of this research through the Prolific Research Group, Project No. PRG-1436-18.

References

1. T. Abbasi and S. A. Abbasi, Renewable Sustainable Energy Rev., 2011, 15, 3034–3040.
2. S. U. M. Khan, M. Al-Shahry and W. B. Ingler, Science, 2002, 297, 2243–2245.
3. V. Artero, M. Chavarot-Kerlidou and M. Fontecave, Angew. Chem., Int. Ed., 2011, 50, 7238–7266.
4. P. Zegers, J. Power Sources, 2006, 154, 497–502.
5. H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan and P. Strasser, ChemCatChem, 2010, 2, 724–761.
6. F. A. Frame, T. K. Townsend, R. L. Chamousis, E. M. Sabio, T. Dittrich, N. D. Browning and F. E. Osterloh, J. Am. Chem. Soc., 2011, 133, 7264–7267.
7. A. Mills, P. A. Duckmanton and J. Reglinski, Chem. Commun., 2010, 46, 2397–2398.
8. A. Izgorodin, O. Winther-Jensen and D. R. MacFarlane, Aust. J. Chem., 2012, 65, 638–642.
9. F. Zhou, A. Izgorodin, R. K. Hocking, L. Spiccia and D. R. MacFarlane, Adv. Energy Mater., 2012, 2, 1013–1021.
10. M. Nakayama, A. Tanaka, Y. Sato, T. Tonosaki and K. Ogura, Langmuir, 2005, 21, 5907–5913.
11. G. F. Swiegers, D. R. MacFarlane, D. L. Officer, A. Ballantyne, D. Boskovic, J. Chen, G. C. Dismukes, G. P. Gardner, R. K. Hocking, P. F. Smith, L. Spiccia, P. Wagner, G. G. Wallace, B. Winther-Jensen and O. Winther-Jensen, Aust. J. Chem., 2012, 65, 577–582.
12. P. Du and R. Eisenberg, Energy Environ. Sci., 2012, 5, 6012–6021.
13. F. E. Osterloh, Chem. Mater., 2007, 20, 35–54.
14. E. Azaceta, S. Chavhan, P. Rossi, M. Paderi, S. Fantini, M. Ungureanu, O. Miguel, H.-J. Grande and R. Tena-Zaera, Electrochim. Acta, 2012, 71, 39–43.
15. M. Wu, Y. Huang, C. Yang and J. Jow, Int. J. Hydrogen Energy, 2007, 32, 4153–4159.
16. F. Vera, R. Schrebler, E. Muñoz, C. Suarez, P. Cury, H. Gómez, R. Córdova, R. E. Marotti and E. A. Dalchiele, Thin Solid Films, 2005, 490, 182–188.
17. X. Hou, J. Williams and K.-L. Choy, Thin Solid Films, 2006, 495, 262–265.
18. R. Cerc Korosec and P. Bukovec, Acta Chim. Slov., 2006, 53, 136–147.
19. A. El-Shaer, A. R. Abdelwahed, A. Tawfik, M. Mossad and D. Hemada, International Journal of Emerging Technology and Advanced Engineering, 2014, 4, 595–602.
20. G. Wu, N. Li, D.-R. Zhou, K. Mitsuo and B.-Q. Xu, J. Solid State Chem., 2004, 177, 3682–3692.
21. M. Gong and H. Dai, Nano Res., 2015, 8, 23–39.
22. A. K. M. Fazle Kibria and S. A. Tarafdar, Int. J. Hydrogen Energy, 2002, 27, 879–884.
23. G. Wu, N. Li, D.-R. Zhou, K. Mitsuo and B.-Q. Xu, J. Solid State Chem., 2004, 177, 3682–3692.
24. E. B. Castro and C. A. Gervasi, Int. J. Hydrogen Energy, 2000, 25, 1163–1170.
25. I. J. Godwin, R. L. Doyle and M. E. G. Lyons, J. Electrochem. Soc., 2014, 161, F906–F917.
26. P. Moller, J. B. Rasumussen, S. Kohler and L. P. Nielsen, NASF, 2013, 78, 15–24.
27. E. Azaceta, N. T. Tuyen, D. F. Pickup, C. Rogero, J. E. Ortega, O. Miguel, H.-J. Grande and R. Tena-Zaera, Electrochim. Acta, 2013, 96, 261–267.
28. A. C. Sonavane, A. I. Inamdar, P. S. Shinde, H. P. Deshmukh, R. S. Patil and P. S. Patil, J. Alloys Compd., 2010, 489, 667–673.
29. A. Singh, S. L. Y. Chang, R. K. Hocking, U. Bach and L. Spiccia, Catal. Sci. Technol., 2013, 3, 1725–1732.
30. Y. Zhao, R. Nakamura, K. Kamiya, S. Nakanishi and K. Hashimoto, Nat. Commun., 2013, 4, 1–7.
31. A. C. Sonavane, A. I. Inamdar, P. S. Shinde, H. P. Deshmukh, R. S. Patil and P. S. Patil, J. Alloys Compd., 2010, 489, 667–673.
32. Y. Sato, M. Ando and K. Murai, Solid State Ionics, 1998, 113, 443–447.
33. S. Z. F. Rodzi and Y. Mohd, in Humanities, Science and Engineering Research (SHUSER), 2012 IEEE Symposium, 2012, pp. 531–535.
34. K.-W. Nam and K.-B. Kim, J. Electrochem. Soc., 2002, 149, A346–A354.
35. D. Mohammadyani, S. A. Hosseini and S. K. Sadrnezhaad, Int. J. Mod. Phys.: Conf. Ser., 2012, 05, 270–276.
36. K. J. A. raj and B. Visnathan, Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem., 2011, 50, 176–179.
37. F. Zhou, A. Izgorodin, R. K. Hocking, V. Armel, L. Spiccia and D. R. MacFarlane, ChemSusChem, 2013, 6, 643–651.
38. H.-Y. Wu and H.-W. Wangint, J. Electrochem. Sci. Eng., 2012, 7, 4405–4417.
39. M. L. Dos Santos, R. C. Lima, C. S. Riccardi, R. L. Tranquilin, P. R. Bueno, J. A. Varela and E. Longo, Mater. Lett., 2008, 62, 4509–4511.
40. S. Mochizuki, Phys. Status Solidi B, 1984, 126, 105–114.
41. A. Šurca, B. Orel and B. Pihlar, J. Solid State Electrochem., 1998, 2, 38–49.
42. I. Zhitomirsky and A. Petric, Mater. Lett., 2000, 46, 1–6.
43. X. Pang, I. Zhitomirsky and M. Niewczas, Surf. Coat. Technol., 2005, 195, 138–146.
44. R. S. Patil, M. D. Uplane and P. S. Patil, Appl. Surf. Sci., 2006, 252, 8050–8056.
45. B. H. R. Suryanto, X. Lu, H. M. Chan and C. Zhao, RSC Adv., 2013, 3, 20936.
46. F. Zhou, A. Izgorodin, R. K. Hocking, V. Armel, L. Spiccia and D. R. Macfarlane, ChemSusChem, 2013, 6, 643–651.
47. S. Trassati, Electrochim. Acta, 1984, 29, 1503.
48. B. S. Yeo and A. T. Bell, J. Am. Chem. Soc., 2011, 133, 5587–5593.
49. M. E. G. Lyons and M. P. Brandon, Int. J. Electrochem. Sci., 2008, 3, 1386–1424.
50. J. McBreen, in Handbook of Battery Materials, Wiley-VCH Verlag GmbH, 2007, pp. 135–151.
51. P. W. T. Lu and S. Srinivasan, J. Electrochem. Soc., 1978, 125, 1416.
52. I. M. Sadiek, A. M. Mohammad, M. E. E. Shakre, M. S. E.-D. M. Ismail Awad and E. El-Anadouli, Int. J. Electrochem. Sci., 2012, 7, 3350–3361.

---